The next afternoon, Nunes called a news conference in the Capitol, where he announced that “sources” had provided him with evidence that, during the transition, the president or his associates had been swept up in foreign surveillance by American spy agencies. When a reporter asked him if he thought that the American intelligence community was “spying on Trump during the transition,” Nunes replied, “I guess it all depends on one’s definition of spying.” He added: “Clearly it bothers me enough. I’m not comfortable with it.” Nunes seemed to be lending credence to Trump’s preposterous accusation on Twitter from earlier that month that Obama had his “ ‘wires tapped’ in Trump Tower.”

Nunes then headed back to the White House to brief Trump on his findings — a seemingly unnecessary move considering that the information had been given to him by Trump’s own aides. A few days later, after reporters exposed the ruse (including the fact that the Obama-era eavesdropping had been legal, incidental and inconsequential), Nunes recused himself from the committee’s Russia investigation, although he still refused to cede subpoena power to his replacement, Mike Conaway. In December, after the House Ethics Committee concluded that Nunes hadn’t illegally revealed classified information, he more firmly took the reins of the Russia investigation.

For House Democrats, Nunes’s “midnight run,” as they now call it, represented a fundamental break. “Devin and I had a very good relationship until March 21,” Schiff told me. “From that point on, I think that he considered it his primary mission to protect the White House no matter the cost.” In the process, Nunes has all but destroyed what was once the House Intelligence Committee’s greatest asset. When the committee was being created in 1977, to exercise legislative oversight of American intelligence agencies, Speaker Tip O’Neill pledged, “This is a nonpartisan committee; there will be nothing partisan about its deliberations.” Although that goal was occasionally tested, the spirit of nonpartisanship generally prevailed and at times even flourished, as it had under Nunes’s predecessor, Mike Rogers, and his Democratic counterpart, Representative Dutch Ruppersberger of Maryland. “It’s not like Dutch and Mike weren’t stalwarts of their own parties, but they knew they had a national security mission,” says Jamil Jaffer, a Republican lawyer who was a senior counsel on the committee. “They got together and said, ‘Look, this stuff is too important to screw up.’ ”

But since Nunes’s midnight run, the committee has been crippled by partisan fighting. When the committee met in late March to discuss releasing the Republicans’ incomplete report on Russia, not one Democrat on the committee voted to do so. Representative Mike Quigley, an Illinois Democrat, openly mocked his Republican colleagues, clucking at them like a chicken. Now that the committee’s Russia investigation is essentially over — Schiff and his fellow Democrats have pledged to keep investigating on their own, but they won’t get far without subpoena power — the ill will isn’t likely to dissipate. “I don’t know that it can recover, given the degree of bad blood that’s developed between the members,” Representative Tom Cole says. Eric Swalwell, a California Democrat and member of the committee, told me: “It is irreparable as long as Devin Nunes is the chairman. He is Trump’s Michael Cohen in Congress. He is Trump’s fixer.” Beyond that, Nunes has damaged the relationship between the committee and the intelligence agencies themselves. “A lot of your effectiveness in overseeing the intelligence community is based on them wanting to have a good relationship with the overseers,” says a former Intelligence Committee staff member. “That’s all blown up. They’re not going to do anything they don’t absolutely have to do. They’re going to marginalize the community.” The former staff member adds: “It’s totally toxic. It’s irreversible. I think the committee has been essentially rendered useless.”

From the ashes of his own committee, though, Nunes has emerged in a far more powerful position. His congressional district is so sufficiently red that despite his Democratic challenger’s multiple appearances on MSNBC, his seat should be safe this November, even in what’s shaping up to be a strong Democratic year. In fact, his stature in the area has been enhanced by the Russia controversy. Jim Brulte, the chairman of the California G.O.P., told me: “If you’re a Republican officeholder in California, and Nancy Pelosi attacks you, I think most Republican voters turn around and go, ‘Wow, I’ve never heard of this guy, but if Pelosi is against him, I’m for him.’ ”

More important, the Russia brouhaha has elevated Nunes far beyond the Central Valley. Once considered the scourge of the Tea Party, he is now viewed as a rock star by the activist Republican base across the country. The National Republican Congressional Committee, the campaign arm of the House G.O.P., uses him in its fund-raising emails. And among his Republican colleagues on Capitol Hill, he has never been more popular — so much so that more than one prominent Republican with whom I spoke predicted that Nunes is on track to one day become the top Republican on the all-powerful Ways and Means Committee.

In February, at its annual Conservative Political Action Conference outside Washington, the American Conservative Union presented Nunes with its Defender of Freedom Award for “concerted courage, standing up for truth and freedom under intense duress.” When Nunes accepted the award, he was greeted with cries of “We love you, Devin!” His familiar grimace gave way to a broad smile.