Although he is well known in the intelligence community, Mr. McCabe, a veteran F.B.I. agent, has little name recognition outside of Washington. He has dealt with Congress for years, but usually behind closed doors.

A Duke University graduate with a law degree from Washington University in St. Louis, Mr. McCabe is considered a skilled briefer. He is something of wunderkind at the F.B.I., rising quickly in the organization to become deputy director in early 2016. But he is not universally loved among agents.

Some inside the F.B.I. believe he did not work in the field long enough and rose too quickly to appreciate the concerns of agents.

Also testifying are Mike Pompeo, the director of the C.I.A.; Dan Coats, the director of national intelligence; Adm. Michael S. Rogers, the head of the National Security Agency; and others.

— Adam Goldman

Democrat seeks warning of political interference.

Mr. Warner, the top Democrat on the committee, sought assurances from Mr. McCabe that he would sound the alarm if the White House or others tried to intervene in the investigation into whether any of Mr. Trump’s associates colluded with Russia in its interference in the election.

“While we don’t know how long you will be acting as F.B.I. director, my first questions for you, even in this public setting, will be for you to assure the committee that if you come under any political influence from the White House or others to squash this investigation, or impede it in any way, that you will let this committee know,” said Mr. Warner, who has called the committee’s own inquiry into Russian election interference “probably the most important thing I’ve done in public life.”