A House candidate in Connecticut dropped out of today’s primary after being arrested.

Joseph R. Biden Jr., who has selected Senator Kamala Harris of California as his vice-presidential running mate, will appear with her in Delaware on Wednesday afternoon where they will deliver remarks. The two will also take part in a virtual grass-roots fund-raiser Wednesday evening.

As his former rival who sharply criticized him in the Democratic primaries, Ms. Harris emerged as a vocal supporter of Mr. Biden after ending her own campaign and has become a prominent advocate of racial-justice legislation after the death of George Floyd while in police custody in late May.

Ms. Harris, 55, is the first Black woman and the first person of Indian descent to be nominated for national office by a major party, and only the fourth woman in history to be chosen for one of their presidential tickets. She brings to the race a far more vigorous campaign style than Mr. Biden’s, including a gift for capturing moments of raw political electricity on the debate stage and elsewhere, and a personal identity and family story that many find inspiring.

Mr. Biden announced the selection over text message and in a follow-up email to supporters: “Joe Biden here. Big news: I’ve chosen Kamala Harris as my running mate. Together, with you, we’re going to beat Trump.”

After her own presidential bid disintegrated last year, many Democrats regarded Ms. Harris as all but certain to attempt another run for the White House in the future. By choosing her as his political partner, Mr. Biden may well be anointing her as the de facto leader of the party in four or eight years.

Ms. Harris, a pragmatic moderate who spent most of her career as a prosecutor, was seen throughout the vice-presidential search as among the safest choices available to Mr. Biden. She has been a reliable ally of the Democratic establishment, with flexible policy priorities that largely mirror Mr. Biden’s, and her supporters argued that she could reinforce Mr. Biden’s appeal to Black voters and women without stirring particularly vehement opposition on the right or left.

After leaving the presidential race in December, Ms. Harris turned her attention back to the Senate and found new purpose amid a wave of nationwide protests this spring against racism and police brutality. She marched beside protesters and forcefully championed proposals to overhaul policing and make lynching a federal crime, often speaking with a kind of clarity that had eluded her in the presidential primaries on economic issues like health care and taxation.

WILMINGTON, Del. — As progressive insurgents have notched primary victories in House and Senate races from New York to St. Louis to Tennessee in recent weeks, showcasing the energy on the left wing of the Democratic Party, Mr. Biden’s selection of Ms. Harris represents a decision instead to double down on his own, relatively centrist instincts.

While Ms. Harris certainly embraced a number of more liberal policy positions than Mr. Biden did in the presidential primary, she is also a former prosecutor who is associated with the establishment wing of the Democratic Party, and was never a favorite of the most progressive activists during her own presidential bid.

Even as Mr. Biden and Ms. Harris have themselves moved to the left on some issues, the Democratic ticket now appears designed to deepen Mr. Biden’s appeal with some of his strongest constituencies — moderate Black voters overall, college-educated women and white suburbanites, for example — rather than a concerted effort to engage the younger, more progressive voters of all backgrounds who have been most skeptical of Mr. Biden.

The challenge for the Biden-Harris ticket now will be to strengthen Mr. Biden’s appeal to a broad coalition while finding more ways to invigorate ascendant liberal forces in their party.

The online messaging wars to define a new running mate are quick and fierce. Just as Democrats moved to shore up a unified support of the selection of Ms. Harris, allies of Mr. Trump were quick to criticize the newly minted running mate as overly liberal and pointed to her primary campaign clashes with Mr. Biden.

Senator Lindsey Graham, while calling Ms. Harris “smart,” said she was also “aggressive” and that she “fully bought in to the Democratic Party’s very liberal agenda.” Representative Liz Cheney, who occasionally has battled with Mr. Trump, claimed Ms. Harris would “take away guns & health insurance.” Ronna McDaniel, the chairwoman of the Republican National Committee, said Ms. Harris had “extreme positions.”

The president’s son, Donald Trump Jr., tried to claim that Ms. Harris would be a shadow president in a Biden administration, and Bill Bennett, a conservative pundit, said on Fox News that “I can’t think of a time when the announcement of the V.P. pick overshadowed the presidential pick.” (That, of course, is impossible because a “presidential pick” is actually an election, not a personal choice.)

Conservative media outlets like the Daily Caller were quick to highlight clips from the Democratic primary debates last year, in which Ms. Harris and Mr. Biden clashed several times. America Rising, a Republican research group, began publishing a Twitter thread attempting to poke holes in Ms. Harris’s record.

The president went on one of his favorite conservative shows — “Hannity” — to discuss the selection of Ms. Harris. Though he often strayed from talking about the selection, Mr. Trump was quick to attack Ms. Harris on her record on guns and painting her as broadly liberal, before criticizing her failed run for the nomination.

“She was one of the people that was projected to have a chance at winning, and all she did as people got to know her, she went down,” Mr. Trump said. “She finished at two percent, probably less than that. And she fled, remember how quickly?”

The president also brought up past statements Ms. Harris made that were critical of Mr. Biden, including on race and on complaints from women about unwanted touching.

“She was my number one draft pick,” Mr. Trump said. “She was the one I thought would be the best for us. Mike will handle her very easily, Mike is a pro.”

President Trump wasted little time assailing Senator Kamala Harris of California on Tuesday after her selection as the Democratic Party’s vice-presidential candidate, calling her “nasty” and “just about the most liberal person in the U.S. Senate.”

Mr. Trump’s campaign quickly posted a video ad attacking her as a “phony” who would pull Joseph R. Biden Jr. to the left and would be the one who really pulled the strings in a Democratic administration. At a news briefing shortly after the announcement, Mr. Trump attacked Ms. Harris for supporting tax increases and expanding Medicare, suggesting she would be an easy target.

“She was my No. 1 pick,” Mr. Trump said. “She did very, very poorly in the primaries, as you know. She was expected to do well. And she ended up right around 2 percent. Spent a lot of money. She had a lot of things happening. I was a little surprised he picked her.”

He described her four times as “nasty” or “nastier,” one of his favorite terms for female opponents, complaining that she had not been nice to his Supreme Court nominee, Brett Kavanaugh, during confirmation hearings and noting that she had attacked Mr. Biden during their contest for the Democratic nomination.

“She was very, very nasty,” Mr. Trump said. “She was very disrespectful to Joe Biden. It is hard to pick somebody that is disrespectful. She said things during the debate, during the Democrat primary debates that were horrible about Sleepy Joe. And I would think that he would not have picked her.”

He also said of Mr. Biden, “He is handing over the reins to Kamala while they jointly embrace the radical left.”

On Tuesday, the Biden campaign sent a warning shot ahead of a selection that many Democrats feared would lead to attacks aimed at whichever woman Mr. Biden ultimately chose.

Mr. Biden’s campaign seized on a statement from a spokeswoman for the Trump campaign in which she said that the former vice president had sold “his soul to the radical left” and asserted that his running mate, whomever it ends up being, would do the same.

Joseph R. Biden Jr.’s campaign said on Tuesday that the announcement of Senator Kamala Harris as his running mate had ignited the biggest online fund-raising hour of his campaign as Tuesday became its biggest overall grass-roots fund-raising day, with millions of dollars pouring into his campaign coffers.

The campaign did not disclose how much it had raised.

But ActBlue, the main donation-processing platform for Democrats, reported more than $10.8 million in donations between 4 p.m. and 8 p.m. on Tuesday afternoon.

The day before, ActBlue had processed $2.3 million during those same four hours, suggesting a bump as large as $8.5 million for Mr. Biden.

It was the largest and most diverse Democratic presidential field in history, with early debates having to be spread out across multiple nights and the Democratic National Committee having to raise debate qualifying standards to start to winnow the field.

But many of the former Oval Office aspirants were in agreement on Tuesday as they lauded the selection of Ms. Harris to be Mr. Biden’s running mate, pointing to certain positions that fit with their core platforms. Senator Bernie Sanders pointed to Ms. Harris’s support of “working people,” Senator Kirsten Gillibrand called her a “fearless champion for justice,” and Senator Elizabeth Warren noted her previous battles with Wall Street.

Senator Cory Booker, who painted Ms. Harris’s exit from the race in December in personal terms, called his friend a “trailblazer.” Julian Castro referred to the selection as historic and Ms. Harris as a “groundbreaking leader.” Pete Buttigieg noted that she “fights tirelessly for justice, dignity, and equality.”

And Mike Bloomberg framed the choice as one a smart businessman would make: “The best executives know that you’re only as good as the team you build.”

Several of the women who were on Mr. Biden’s running mate short list but were not selected quickly began to coalesce around the party’s ticket after the former vice president announced that he had picked Ms. Harris.

Susan E. Rice, the former national security adviser to President Barack Obama, vowed to work on behalf of the ticket after learning that she had not been selected as Mr. Biden’s running mate.

“As I have said from the outset, I will do my utmost to assist Joe Biden to become the next president of the United States and to help him govern successfully,” Ms. Rice wrote on Twitter.

Representative Karen Bass of California commended Mr. Biden on his selection of Ms. Harris.

“Her tenacious pursuit of justice and relentless advocacy for the people is what is needed right now,” Ms. Bass tweeted.

Senator Elizabeth Warren of Massachusetts cited Ms. Harris’s role during the 2008 financial crisis as a moment of strength for her Senate colleague.

The selection of Senator Kamala Harris as the Democratic nominee for vice president marks the latest evidence that gender and race have now surpassed geographic balance when it comes to building a ticket for the White House.

Ms. Harris, whose father is Black and whose mother is Indian-American, is not expected to scramble the electoral map, nor was the Biden campaign looking to do so. The former vice president leads in polls of most of the crucial battlegrounds.

Instead, ever since Black voters resurrected his primary candidacy in South Carolina, Mr. Biden and his campaign team have made the pursuit of Black voters in November a centerpiece of his bid for the White House. And he had said from the start of the process that he would choose a woman as his running mate.

Robert Shrum, director of the Center for the Political Future at the University of Southern California, said more than anything the choice of Ms. Harris should address one of the reasons Ms. Clinton lost to Mr. Trump so narrowly in 2016.

“One of the problems in 2016 was a fall-off in the African-American vote in Wisconsin, Michigan and Pennsylvania,” he said.

“She is going to be a great motivator for this ticket,” Representative James Clyburn of South Carolina, a key Biden endorser, said.

“This is 1,000 percent a demographic selection,” said Theodore R. Johnson, a senior fellow at the Brennan Center for Justice, who studies voting behavior among Black voters. He predicted that Ms. Harris would increase Black turnout.

Many Democrats celebrated the history-making choice on Tuesday — no Black woman or woman of color has ever been nominated for vice president or president on a major ticket.

“This has sent a lightning bolt of electricity across a base that has been watching and waiting and looking for a reason to be excited about this race,” said Matt Morrison, the executive director of Working America, a labor-backed political group with three million members. “I have a Black mother who is literally through the roof, and she is emblematic of the visceral excitement of the base that drove Barack Obama to the White House.”

Representative Ilhan Omar of Minnesota fended off a well-funded primary challenger on Tuesday, ensuring a clean sweep of re-election fights for the group of first-term Democratic congresswomen of color known as the Squad and sending a message to Washington about the staying power of the party’s new progressive voices.

Ms. Omar, who made history in 2018 by becoming the first Somali-American to be elected to Congress, as well as the first naturalized citizen of African birth and the first woman of color from Minnesota to do so, secured the victory after spending her first two years in the Washington spotlight.

Her unabashed embrace of left-wing politics has won her loyal followers, both in Minnesota and across the country. She has, however, become a lightning rod for conservatives and has faced criticism from some Democrats, particularly after several episodes in 2019 in which she was accused of making anti-Semitic remarks.

In the deep-blue district, Ms. Omar’s primary success on Tuesday will virtually assure she serves a second term in Congress.

Antone Melton-Meaux, Ms. Omar’s chief opponent, had received extensive financial support from national groups hoping to see the incumbent ousted. Still, Ms. Omar prevailed. The race was more than a referendum on her first term, and was upended by the two factors that have reshaped every facet of political life in 2020: the coronavirus pandemic and the national protests over racial inequality and police brutality that began in May after George Floyd was killed in Minneapolis. The unrest was particularly acute in Minneapolis, where fires and chaos raged for days. One Police Department precinct was burned to the ground.

Conspiracy theorists won a major victory on Tuesday as Marjorie Taylor Greene, a Republican supporter of the convoluted pro-Trump movement QAnon, triumphed in her House primary runoff election in Georgia, all but ensuring that she will represent a deep-red district in Congress.

The ascension of Ms. Greene, who embraces a conspiracy theory that the F.B.I. has labeled a potential domestic terrorism threat, is likely to unsettle mainstream Republicans, who have sought to publicly distance themselves from QAnon supporters running for congressional office this cycle even as they quietly support some of them.

Ms. Greene defeated John Cowan, a neurosurgeon who is no less conservative or pro-Trump, in Georgia’s 14th Congressional District, one of the most Republican in the country. QAnon, a conspiracy theory that has attracted a fervent following since it emerged from the troll-infested fringes of the internet nearly three years ago, has already inspired real-world violence, including the killing of a mob boss. Its supporters are slowly becoming a political force that some Republicans feel they cannot afford to alienate, even as the party struggles to distance itself from racist and anti-Semitic conspiracy theories.

More than a dozen candidates who have expressed some degree of support for QAnon are running this year for Congress as Republicans, their path cleared by Mr. Trump’s own espousal of conspiracy theories. Most are going to lose. But a few, Ms. Greene foremost among them, have managed to win primaries against Republicans whose only real ideological difference was that they don’t believe in QAnon.

In selecting Senator Kamala Harris of California to be his running mate, Joseph R. Biden Jr. opted for a partner who has occupied a similar space in the center-left of the ideological spectrum — someone with whom he can most likely agree on many matters of policy.

Amid a field of more than 20 candidates in the Democratic primary, Mr. Biden and Ms. Harris were seen as a relative moderates. During her own unsuccessful presidential campaign, Ms. Harris often resisted litmus tests posed within Democratic circles and at times struggled to clearly define her stance on fundamental issues like whether she would endorse a “Medicare for all”-style of health insurance.

Here is a look at where Ms. Harris stands on key policy issues.

Racial justice and policing

Ms. Harris is a former prosecutor, and her handling of cases involving police shootings of civilians while she was California’s attorney general drew criticism from activists on the left, who argued that she was not aggressive enough in stepping in to overhaul rogue police departments, and that she sided too frequently with police unions.

As a Democratic presidential candidate, Ms. Harris released plans outlining her vision for criminal justice reform and how to “stand up for Black America.” As part of the plans Ms. Harris called for ending mass incarceration, cash bail and the death penalty; creating a national police systems review board; making attending historically Black colleges and universities debt-free for students; and many other measures.

Perhaps the most memorable moment of her presidential campaign came during the first round of Democratic primary debates, when Ms. Harris confronted Mr. Biden about his fond recollections of working with segregationists in the Senate and then opened up about her personal history with school busing.

Mr. Biden, she said, had opposed school busing mandates to integrate schools in the 1970s.

In more recent months, since the death of George Floyd in Minneapolis, Ms. Harris has spoken out strongly about the injustice faced by Black people in the United States. She introduced police accountability legislation known as the Justice in Policing Act this summer and sparred with Senator Rand Paul, Republican of Kentucky, after he blocked a bill to make lynching a federal crime.

Health care

In the Senate, Ms. Harris co-sponsored Senator Bernie Sanders’s Medicare for all legislation, and at a CNN town hall event early in her campaign, she responded to a question about private health insurance by saying, “Let’s eliminate all of that.”

Then, on the debate stage, the Democratic candidates were asked who would abolish private health insurance. Ms. Harris was among those who raised their hands. But after the debate, she said she had misunderstood the question.

Ms. Harris later put out her own health care plan, which placed her somewhere on the ideological spectrum between Mr. Biden and Mr. Sanders. Her plan sought to provide Medicare for all Americans, but it kept a significant role for health insurance companies by allowing people to choose private plans modeled on Medicare Advantage.

MESA, Ariz. — Vice President Mike Pence accused Mr. Biden on Tuesday of selling out to the liberal wing of the Democratic Party after Mr. Biden’s big reveal of his running mate: Senator Kamala Harris.

Speaking to a group of Mormon supporters in Mesa, Ariz., Mr. Pence said that Mr. Biden and Ms. Harris would lead the country on a path of “socialism and decline.”

“Joe Biden and the Democratic Party have been overtaken by the radical left,” Mr. Pence said.

Mr. Pence’s criticism of the opposing ticket marked an opening salvo of the vice-presidential matchup between himself and Ms. Harris, who is known as a skilled and prosecutorial debater. It also highlighted the traditional role of the vice-presidential candidates as attack dogs.

“Given their promises of higher taxes, open borders, socialized medicine and abortion on demand, it’s no surprise he chose Senator Harris,” Mr. Pence said.

Former President Barack Obama on Tuesday offered accolades for his onetime political partner, saying that Mr. Biden had made a pragmatic decision by naming Senator Kamala Harris of California as his running mate.

“Joe Biden nailed this decision,” Mr. Obama said in a statement. “By choosing Senator Kamala Harris as America’s next vice president, he’s underscored his own judgment and character. Reality shows us that these attributes are not optional in a president. They’re requirements of the job.”

Mr. Obama’s praise of Mr. Biden’s choice was echoed by a number of the Democratic Party’s current and former standard bearers.

Hillary Clinton, the party’s 2016 presidential nominee and a former secretary of state, tweeted that the Biden-Harris ticket was “historic” and that Ms. Harris was “an incredible public servant and leader.”

House Speaker Nancy Pelosi referred to Mr. Biden’s selection of Ms. Harris as a proud milestone for the country.

“As a United States Senator, as California Attorney General and in every leadership position that she has held, Kamala Harris has advanced a more just and fair future for all,” Ms. Pelosi said in a statement. “Rooted in strong values and her proud American story, Vice President Harris will be a tireless champion for hard-working families everywhere.”

It was a list that felt long until the very end, as new names seemed to keep emerging and former favorites withdrew. But after Mr. Biden made his long-awaited selection of Senator Kamala Harris of California as his running mate, several more women who had once been reportedly considered by the Biden team as a potential choice all lauded the selection of Ms. Harris.

The nearly instant and unanimous praise from the previous vice-presidential contenders underscored how important presenting a united front was for Democrats across the ideological spectrum.

Stacey Abrams jumped in to say she was “thrilled to support” Ms. Harris as Mr. Biden’s running mate, adding that Mr. Biden’s “focus on reaching out to every corner of our country speaks to how he will lead us.”

In a Zoom call with about 200 members of the Oakland County Democratic Party Tuesday evening, Ms. Whitmer said:

“It’s not going to be enough to just win, we’ve got to win decisively. Because what we know from the Trump administration is they’re already working to undermine the election. They’re already working to call into question and make it harder to vote by mail. It’s crazy, it’s ludicrous and incredibly dangerous.”

“I’m really excited about Joe Biden and Kamala Harris and our candidates up and down the ticket. It’s going to be the kind of fierce leadership that we need in Washington, D.C. The key to the White House goes through Michigan and the key to Michigan goes through Oakland County.”

A new national poll released Tuesday shows Mr. Biden maintaining a 10-point lead over Mr. Trump, with just 4 percent of voters remaining undecided.

The poll, conducted by Monmouth University, showed Mr. Biden garnering the support of 51 percent of registered voters and Mr. Trump earning 41 percent. A small share of support went to third-party candidates and the rest were undecided.

Mr. Biden’s lead was about the same as he had in a late-June survey by the same pollster, in which Mr. Biden was ahead of Mr. Trump by 12 percentage points.

The Monmouth Poll was conducted by telephone from Aug. 6 to Aug. 10 and has a margin of error of 3.5 percentage points.

In Wisconsin, a swing state won by Mr. Trump in 2016, Mr. Biden led the president by six percentage points in a Marquette Law School poll of registered voters that was released on Tuesday.

Six percent of those polled said that they would not vote for either Mr. Trump or Mr. Biden, 3 percent were undecided and 1 percent would not disclose their choice for president.

Mr. Trump’s job approval ratings continued to slide in Wisconsin, particularly on his handling of the coronavirus pandemic — 58 percent of those polled said they disapproved of his response to the health crisis. The poll had a margin of error of 3.9 percentage points.

While she was not quite claimed as an honorary citizen, Canadians on social media along with the Canadian news media swiftly recalled Kamala Harris’s connection to their country on Tuesday.

“Could a former Westmount High student be on her way to the White House?” the Montreal Gazette asked in the top item on its website under a headline that described Ms. Harris as “a former Montrealer.” The Canadian Broadcasting Corporation echoed that thought in its headline: “From Westmount High to the White House?”

On Twitter, a user swiftly posted Ms. Harris’s photo from a high school yearbook, her hair much longer than its current styling. And shortly after Mr. Biden made the announcement one man predicted, somewhat accurately if with considerable exaggeration, that there would be an “Incoming wave of ‘KAMALA HARRIS WENT TO SCHOOL IN MONTREAL’ stories from a zillion Canadian journalists.”

Ms. Harris lived in Montreal for about five years and graduated from high school there in 1981 while her mother, Shyamala Gopalan Harris, did breast cancer research at a hospital and lectured at McGill University.

[View live results from Tuesday’s races in Connecticut, Georgia, Minnesota and Wisconsin.]

A Republican running for Congress in Connecticut was arrested Monday night and dropped out of the primary campaign just hours before voters went to the polls on Tuesday, the authorities and state party officials said.

The candidate, Thomas Gilmer, was charged with strangulation and unlawful restraint in connection with a “possible domestic assault,” the police in Wethersfield, a Hartford suburb, said in a statement.

In a post on Twitter, the Connecticut Republican Party said Mr. Gilmer had ended his campaign.

Mr. Gilmer, a businessman, had won the Republican Party’s endorsement in May but faced a primary challenge today from Justin Anderson, a lieutenant colonel in the National Guard. In November, the primary winner will take on the longtime Democratic incumbent, Representative Joe Courtney, who was re-elected by a 62-to-35-percent margin in 2018.

Mr. Gilmer, 29, could not be immediately reached for comment Tuesday morning.

The police said they were contacted in July about the episode that led to Mr. Gilmer’s arrest. The authorities did not provide any additional details.

Gabe Rosenberg, a spokesman for Connecticut’s Secretary of State, said the office had not received formal notice of Mr. Gilmer’s withdrawal from the race as of Tuesday morning.

Thousands of absentee ballots have already been mailed out, Mr. Rosenberg said, and if Mr. Gilmer wins Tuesday’s primary, he would remain on the November ballot unless he formally withdraws.

Republicans might be able to nominate someone to replace Mr. Gilmer if he wins and withdraws, depending on the timing, Mr. Rosenberg said.

The Democratic National Convention will play out like a star-studded Zoom call next week, anchored by nightly prime-time keynote speeches, with Michelle Obama appearing on Monday, Jill Biden on Tuesday, Barack Obama on Wednesday, and Mr. Biden’s acceptance speech on Thursday, according to a schedule of events.

The convention, originally planned for Milwaukee, then forced into a cramped virtual format by the coronavirus, has been a logistical nightmare for planners who have had to grapple with wary television networks, daunting technical challenges and the omnipresent, low-grade threat of a disruption by Mr. Trump.

The schedule, provided by Democratic officials involved in the planning, above all else reflects Mr. Biden’s chief political goal: uniting the jostling progressive and establishment wings of the Democratic Party behind an elder statesman who has spent the last several months courting skeptical progressives.

The first-night schedule reflects that big-tent objective. Senator Bernie Sanders of Vermont, Mr. Biden’s main rival for the nomination — and still the standard-bearer of the populist left — has been given a keynote slot, just before Mrs. Obama speaks, and after Andrew M. Cuomo, the moderate governor of New York, delivers what is expected to be a scathing attack on Mr. Trump’s handling of the pandemic.

After the formality of a virtual delegate vote on Tuesday, Mr. Biden’s running mate, whom he announced will be Kamala Harris, will address the convention on Wednesday.

About three-quarters of all American voters will be eligible to receive a ballot in the mail for the 2020 election — the most in U.S. history, according to a New York Times analysis. If recent election trends hold and turnout increases as experts predict, roughly 80 million mail ballots will flood election offices this fall, more than double the 2016 figure.

The rapid and seismic shift can be traced to the coronavirus pandemic. Concerns about virus transmission at polling places have forced many states to make adjustments on the fly that — despite President Trump’s protests — will make mail voting in America more accessible this fall than ever before.

“I have a hard time looking back at history and finding an election where there was this significant of a change to how elections are administered in this short a time period,” said Alex Padilla, the California secretary of state and chairman of the Democratic Association of Secretaries of State.

Most of the changes are temporary and have been made administratively by state and local officials, using emergency powers. Over all, 24 states and the District of Columbia have in some way expanded voter access to mail ballots for the 2020 general election.