Mr. Ryan, for his part, waved off any suggestion of removing Mr. Nunes.
“He’s focusing on keeping our country safe, focused on national security,” he told reporters here at the Greenbrier resort, where congressional Republicans are gathered for their annual retreat. He accused Democrats of playing “some political game” to divert attention from tax cuts and a strengthening economy.
A spokesman for Mr. Nunes did not respond to messages seeking comment.
Mr. Nunes, 44, has made an unlikely transition from a leadership loyalist who once heaped scorn on Republican hard-liners to one of the hard-liners’ idols. His “lemmings” comment came in 2013 as Senator Ted Cruz, Republican of Texas, and the conservative House Freedom Caucus were pressing to shut down the government unless President Barack Obama agreed to stop all funding for the Affordable Care Act. At that time, he was quick with a quip to dismiss his party’s impractical right flank.
The Nunes family has run a dairy farm in California’s Central Valley for three generations. At age 23, he ousted a seasoned incumbent on the board of the two-year College of the Sequoias, which he had attended. From that seat, he befriended his representative, Bill Thomas, then a Republican powerhouse in Congress, and a Thomas staff member, Kevin McCarthy, who rose to be the House majority leader.
Elected to the House in 2002 at age 29, he landed a spot on the Ways and Means Committee, courtesy of its retiring chairman, Mr. Thomas. His affinity for Republican leadership served him well. He vied for the chairmanship of the Intelligence Committee in 2014, and proved to be a far better fund-raiser than more senior lawmakers against whom he was competing. That helped persuade then-Speaker John A. Boehner to give Mr. Nunes the job.
But the Trump era has changed a lot of politicians, Mr. Nunes among them. After he signed up for Mr. Trump’s presidential transition, he found advantages. “I have a lot more friends, that’s definitely for sure,” he joked to McClatchy News Service.
The House and Senate Intelligence Committees were created in the 1970s post-Watergate era with a kind of unwritten understanding: The nation’s intelligence agencies would submit to oversight and share sensitive information with selected members of Congress and their staff, and in exchange the committees would operate in a bipartisan fashion — and not use sensitive intelligence for political gain.
Even some of the House’s most hard-charging partisans, from Representative Nancy Pelosi of California on the left to Representatives Mike Rogers of Michigan on the right, have tempered their political instincts on intelligence matters.
That understanding held until about a year ago, when Mr. Nunes — a staunch supporter of Mr. Trump — called reporters on behalf of Mr. Trump in a bid to counter what the president considered negative stories.
Then, in March, as the investigations into Russian meddling in the 2016 election appeared to be picking up momentum, Mr. Nunes set off a bizarre Washington drama when he made a late-night dash to the White House, and followed up with a morning news conference in which he claimed that he had been given intelligence reports that Mr. Trump and his associates were incidentally swept up in foreign surveillance by American spy agencies during the campaign.
Furthermore, Mr. Nunes charged, the identities of the Mr. Trump and his associates swept up by the surveillance, which are supposed to be “masked” in intelligence reports, had been unlawfully revealed in classified reports at the order of senior Obama administration officials.
It turned out that the intelligence cited by Mr. Nunes was given to him by a pair of senior officials at the Trump White House, and that it had selectively cited certain incidents to show wrongdoing where none may have existed. Senator Lindsey Graham, Republican of South Carolina, declared that Mr. Nunes was running an “Inspector Clouseau investigation.”
The incident prompted an ethics investigation and forced Mr. Nunes to recuse himself from the committee’s Russia investigation, a move that Representative Peter T. King, Republican of New York and a committee member, said almost certainly left Mr. Nunes feeling disappointed.
“This is the largest issue facing the Intelligence Committee in years, probably the biggest one since the Iraq war,” Mr. King said, adding, “It’s a moment in history, and you want to be there.”
Mr. Nunes was cleared in December, and is reasserting his authority over the committee, though he has left the day-to-day operations of the Russia investigation to Representative K. Michael Conaway, Republican of Texas. Mr. King said Mr. Nunes participates in some discussions relating to the inquiry, but has not been present when witnesses have testified.
Mr. Nunes has not read the warrant from which the memo is said to be drawn. The Justice Department considers such warrants extremely sensitive and allowed only one Democrat and one Republican from the committee, plus staff, to view it. Rather than do so himself, Mr. Nunes designated Representative Trey Gowdy of South Carolina to be the Republican reader.
Yet even during his recusal, Mr. Nunes appeared eager to turn the spotlight on those who had sought to expose what they believed was a conspiracy to elect Mr. Trump.
Over the summer, he dispatched a pair of Republican staff members working for the committee to London, where they showed up unannounced at the offices of Christopher Steele, the author of a dossier of unsubstantiated information on purported links between the Trump campaign and Russia, and how Russia sought to help Mr. Trump’s candidacy.
The visit was highly unusual; it appeared that the staff members were trying to meet with Mr. Steele outside official channels. Ordinarily, such a visit would be coordinated through lawyers, conducted with knowledge of the committee’s minority party and the American Embassy.
Mr. Nunes once enjoyed a cordial working relationship with his fellow Californian and Democratic counterpart, Representative Adam B. Schiff. But that ended when Mr. Nunes made his run to the White House in March, and Mr. Schiff called for his recusal.
In an interview, Mr. Schiff called Mr. Nunes’s push to release the memo “a cynical strategy that the president applauds,” and said he was “deeply concerned about the breach of faith between our committee and the intelligence community.”
But Republicans were rallying around Mr. Nunes.
“Sometimes the right-wing groups make him out to be the greatest hero that ever lived, and you have all these people demonizing the guy,” Mr. King said. The truth, he said, is somewhere in the middle: “Devin is sort of a regular guy.”