Responsibility for defending the country from cyberattacks is spread across different parts of the government. The Department of Homeland Security is responsible for protecting civilian agencies and advises states, companies and utilities. The F.B.I. investigates cyberattacks.
The N.S.A., which by law can operate only overseas, goes deep into foreign networks but is also responsible for securing national security systems, like communications with the nuclear arsenal. Cyber Command is a military operation responsible for offensive military activity and defending military services from cyberattacks.
Last month, Mr. Trump dismissed Christopher Krebs, the director of the Cybersecurity and Infrastructure Security Agency, part of the Homeland Security Department, for declaring that the presidential election had been one of the best run in United States history. On CNN on Sunday, Mr. Krebs said that he had no inkling of the Russian attack and that American sensors had not been devised to detect this kind of supply-chain intrusion.
All of that would seem to raise questions that Mr. Trump would want to address in his last month. Instead, on Saturday he muddied the intelligence conclusion that the Russians were at the center of the hack, suggesting that it could have been China.
A senior administration official, who spoke on the condition of anonymity to discuss internal deliberations, said that no decisions had been made and that the acting defense secretary, Christopher C. Miller, and his top aides were reviewing the proposal. The official said the Cyber Command proposal was part of a broader review of several defense organization portfolios that Mr. Miller was rushing to complete before Mr. Trump leaves office.
The official said the Pentagon was not acting under pressure from the White House.
Representative Adam Smith, Democrat of Washington and the chairman of the House Armed Services Committee, sent letters to Mr. Miller and General Milley opposing the proposal and warning that any such unilateral action was “not only inadvisable, but is contrary to law.”
A spokesman for General Milley, Col. Dave Butler, said on Saturday, “The chairman has not reviewed nor endorsed any proposal of this kind.”