It was not always thus. Mr. Nunes was 23 when he was first elected to public office, as a board member of a local community college where he had started his higher education. Eyeing his first campaign, Mr. Nunes sought advice from Representative Jim Costa, a Democrat who is also from the Central Valley in California and was serving in the State Legislature at the time.

Both belonged to the area’s tight-knit Portuguese community. Mr. Costa said he recognized a familiar drive. “We all show that kind of interest when we’re in our 20s and 30s, and we want to try to get an opportunity to see if we can make a difference,” Mr. Costa said.

Until recently, Mr. Nunes’s most memorable flourish in the House was his blistering assessment in 2013 of Republicans who were willing to shut down the federal government over President Obama’s health law: “lemmings with suicide vests.”

Now, he has largely retained the support of his colleagues, despite complaints from Republicans like Senator John McCain of Arizona over his committee stewardship, amplifying calls for an independent commission to investigate Mr. Trump’s connections to Russia.

On Friday, Mr. Nunes summoned reporters once more to relay information that Paul Manafort, the former Trump campaign chairman with questionable ties to Russia, had volunteered to appear before the committee. Mr. Nunes also announced the cancellation of a public meeting with former intelligence and law enforcement leaders, citing a desire to bring in James B. Comey, the F.B.I. director, and Michael S. Rogers, the head of the National Security Agency, for a closed session instead. Both testified before the committee this week.

Representative Adam Schiff of California, the committee’s top Democrat, suggested that once again the chairman had acted unilaterally, this time to scuttle the hearing. And in a sign of how far their relationship has fallen, Mr. Schiff — who for weeks stood by Mr. Nunes’s side before reporters and defended him — accused him of taking that action because the White House had told him to.

This was the capper to an eventful few days for Mr. Nunes.

Presiding over a committee hearing on Monday — when Mr. Comey took the extraordinary step of announcing the agency’s investigation into whether the Trump campaign had colluded with Russia to influence the 2016 election — Mr. Nunes mustered deeper alarm over anonymous sources revealing an inquiry’s details to journalists than over the contents of the investigation.