There is little template for a modern F.B.I. chief pursuing a prominent perch in political advocacy. Mr. Comey has performatively bristled at the observation that he is the bureau’s most consequential leader since its first, affecting a grimace recently when a forum moderator introduced him as “the first F.B.I. director since J. Edgar Hoover to be a household name.”

“There’s no precedent,” Tim Weiner, author of “Enemies: A History of the F.B.I.,” said of Mr. Comey’s present ambitions. “But then, there’s never been a president who’s been perceived as a threat to American national security.”

A former registered Republican who planned to complete his 10-year F.B.I. term in 2023, Mr. Comey has urged Democrats against charging to “the socialist left.” He is open to appearing with presidential candidates at campaign functions or even at a nominating convention, if they will have him, though he vowed never to seek office himself.

Mr. Comey donated money earlier this year to Senator Amy Klobuchar of Minnesota, a former law school classmate, but in the interview volunteered few opinions on the rest of the 2020 Democratic field, save for a compliment about the thoughtfulness of the South Bend, Ind., mayor whose name he seemed unsure of (“Is it Pete Boot-ed-edge?”).

Some veterans of law enforcement have questioned the value, and the propriety, of Mr. Comey’s new phase, mocking the emotive turn that has found him posting images of nature and open road on social media.

Rod J. Rosenstein, the former deputy attorney general whose 2017 memo about Mr. Comey was cited to rationalize the firing of the F.B.I. director that May, has been particularly cutting. At a speech last spring, Mr. Rosenstein taunted the former director for “selling books and earning speaking fees while speculating about the strength of my character and the fate of my immortal soul.”