There were times, however, when the preparation eluded Mr. Biden, and Mr. Trump seemed to get under his skin, as his barrage of disruptions and insults were intended to do. Mr. Biden, who overcame a childhood stutter to become a self-assured debater in the Senate, seemed at a momentary loss for words when Mr. Trump attacked his family and questioned his positions on law enforcement.
But more often, Mr. Biden, who has a volatile temper — and was known as a long-winded civic sermonizer during his Senate years — visibly struggled to suppress his anger.
It did not always work — and it did not always work against him. His offhand remark “will you shut up, man?” went viral and was quickly incorporated into his campaign’s branded merchandising operation, even as some detractors viewed his remarks as showing disregard for the office of the president.
Frank Luntz, a Republican pollster who conducted a focus group of undecided voters during the debate, said Mr. Biden’s counterattacks went over the line, and alienated some of the people Mr. Luntz interviewed. “He did the same thing as Trump, just not as badly or as brutally,” he said in an interview. “The loudest voice is sometimes silence. That would have been a much better approach.”
One of the few politically coherent lines of attack pursued by Mr. Trump, 74, was to accuse Mr. Biden of being a Trojan horse for radical leftists — figuring he would either have to hug progressives and therefore alienate moderates, or distance himself, risking a revolt on the left.
The centrist former vice president chose to emphatically distance himself from his party’s progressive wing and threw in a low-key “l’etat, c’est moi” declaration. When the president accused him, falsely, of supporting “socialist” health care proposals, Mr. Biden responded: “The party is me. Right now, I am the Democratic Party,” adding, “The platform of the Democratic Party is what I, in fact, approved of.”
At another point, when Mr. Trump claimed that Mr. Biden “agreed with Bernie Sanders, who is far left,” Mr. Biden batted the remark aside like a bug. “The fact of the matter is: I beat Bernie Sanders,” he said. The president tried to seize on the moment, insisting, “You just lost the left.”