Mr. Rosenstein is the most senior law enforcement official supervising the Russia investigation. Attorney General Jeff Sessions recused himself because of his close ties to the Trump campaign and his undisclosed meetings with Russia’s ambassador to the United States.

At the meeting with the senators, Mr. Comey said he had made the request because he believed the Justice Department had not dedicated enough resources to the investigation, a fact partly stemming from the unusual situation under which the inquiry was being run. Until two weeks ago, when Mr. Rosenstein took over as deputy attorney general, the investigation was being overseen by Dana Boente, who was acting as the deputy and had limited power.



As recently as last week, Mr. Comey said he hoped he would find a supportive boss in Mr. Rosenstein. In congressional testimony last week, Mr. Comey called Mr. Rosenstein “a very independent-minded, career-oriented person” and said he had briefed Mr. Rosenstein on the Russia investigation on his first day in office.

To a president who puts a premium on loyalty, Mr. Comey represented a fiercely independent official who wielded enormous power. But if the White House was hoping Mr. Comey’s firing would provide relief from the pressure of the Russia investigations, the Senate Intelligence Committee appeared eager to fill any temporary void.

Late last month, it asked a number of high-profile Trump campaign associates to hand over emails and other records of dealings with Russians, and the committee’s subpoena of Mr. Flynn on Wednesday made good on its threat to legally compel anyone who failed to voluntarily comply with its request.

Russia’s efforts to meddle in the presidential election are also likely to be a focus of the Senate Intelligence Committee’s annual hearing on worldwide threats on Thursday, which is ordinarily a wider-ranging and policy-focused event.