Top House Democrats on Monday called on the Republican chairman of the House Intelligence Committee to recuse himself from the panel’s investigation into Russian meddling in the 2016 presidential election, thrusting the entire inquiry into jeopardy amid what they described as mounting evidence he was too close to President Trump to be impartial.
The demands followed revelations that the committee’s chairman, Representative Devin Nunes of California, had met on White House grounds with a source who showed him secret American intelligence reports. The reports, Mr. Nunes said last week, showed that Mr. Trump or his closest associates may have been “incidentally” swept up in foreign surveillance by American spy agencies.
The new revelation that the information actually came from a meeting held on the grounds of the White House intensified questions about what prompted Mr. Nunes to make the claim about the intelligence gathering, and who gave him the information.
Representatives Adam B. Schiff of California, the committee’s top Democrat, and Nancy Pelosi of California, the House Democratic leader, suggested that Mr. Nunes, who served on the Trump transition team, was simply too close to the White House to run an independent, thorough inquiry.
“The public cannot have the necessary confidence that matters involving the president’s campaign or transition team can be objectively investigated or overseen by the chairman,” Mr. Schiff said on Monday night.
Still, Mr. Schiff stopped short of pulling the panel’s Democrats out of the investigation. Doing so could jeopardize Democrats’ influence over the inquiry and, importantly, their access to intelligence on possible ties between Trump associates and Moscow.
The House Intelligence Committee is running one of the three investigations into Russian interference in the election, and possible ties between Trump associates and Russia. The Senate Intelligence Committee is running its own inquiry, and the F.B.I. has carried out a broad counterintelligence investigation since July.
By most accounts, the Senate and F.B.I. investigations remain on track, unlike the House inquiry, which appears to have increasingly descended into a sideshow since its first public hearing a week ago. That was when James B. Comey, the director of the F.B.I., publicly disclosed the bureau’s investigation for the first time. Days later, Mr. Nunes made his first disclosure about Mr. Trump or his associates being caught in American intelligence gathering, prompting critics to argue that he was trying to shift attention and provide an assist to the White House at a crucial moment.
The revelation that Mr. Nunes had viewed intelligence materials on White House grounds the day before bolstering the administration’s case fueled damaging speculation that he was acting at the instruction of the president. That could prove fatal to the bipartisan investigation, which has hinged on the ability of Mr. Nunes to conduct a neutral inquiry while maintaining the trust and cooperation of Mr. Schiff.
Ms. Pelosi echoed Mr. Schiff’s call for Mr. Nunes to recuse himself, saying his behavior had “tarnished” his post and urging Speaker Paul D. Ryan to speak out.
“Speaker Ryan must insist that Chairman Nunes at least recuse himself from the Trump-Russia investigation immediately,” she said in a statement. “That leadership is long overdue.”
In an apparent attempt to change the subject, Mr. Trump on Monday night questioned why the House Intelligence Committee is not looking into connections between Hillary Clinton and Russian officials.
A few minutes later, he posted a second message on Twitter, concluding, “Trump Russia story is a hoax.”
The spokesman for Mr. Nunes, Jack Langer, said the congressman met with his source at the White House because he needed access to a secure location where people with security clearances can legally view classified information. But such facilities can also be found in the Capitol building, and at other locations across Washington.
Senator Mark R. Warner of Virginia, the Democratic vice chairman of the Senate Intelligence Committee, called it “more than suspicious” that Mr. Nunes went to the White House complex, pointing out that he would “have to be escorted” while there.
“Who is he meeting with?” Mr. Warner said in an interview with NBC. “Was it a source or somebody from the administration?”
Mr. Langer did not address those concerns on Monday. In a brief statement, he said: “Chairman Nunes met with his source at the White House grounds in order to have proximity to a secure location where he could view the information provided by the source.”
He added, “The chairman is extremely concerned by the possible improper unmasking of names of U.S. citizens, and he began looking into this issue even before President Trump tweeted his assertion that the Trump Tower had been wiretapped.”
Sean Spicer, the White House press secretary, said on Monday that White House officials had no previous knowledge of Mr. Nunes’s visit to the White House grounds, saying the only information he had came from “public reports.”
He also said officials were “not concerned” about the prospect that someone within the executive branch had leaked classified information to Mr. Nunes.
Senator Chuck Schumer of New York, the Democratic leader, accused Mr. Nunes of weakening not only the committee’s tradition of bipartisanship but also Congress itself. He asked Mr. Ryan to replace Mr. Nunes.
“He has not been cooperating like someone who is interested in getting to the unvarnished truth,” Mr. Schumer said.
Acknowledging that the incidental collection from surveillance appeared to be legal, Mr. Nunes last week said his concerns surrounded additional names that may have been improperly “unmasked.” Normally, intelligence agencies mask the identities of American citizens who are incidentally present in intercepted communications.
Mr. Schiff said that Mr. Nunes also worried that anyone viewing the distributed reports could decipher whom they were discussing even though the names were masked.
Mr. Nunes repeatedly declined to offer any details about the source of what he characterized as “dozens” of classified intelligence reports, which Mr. Schiff accused him of viewing in a “dead-of-night excursion.” Mr. Nunes said only that the information had come to him after the committee’s public hearing on Monday.
On Friday, Mr. Nunes declined to say whether that information had come from the White House.
“You can ask me every single name that exists on the planet, and I’m still not going to tell you who our sources are,” he told reporters.
Mr. Nunes then defended his decision to bypass Mr. Schiff and go to the White House, saying he felt a “duty” to tell Mr. Trump because of Democrats’ “relentless” political attacks.
“If we would have crossed paths in the hall, maybe I would have said something to him,” Mr. Nunes said in an interview. “But what I was trying to do was get to the president as quick as possible.”
At that point, Mr. Trump seized on the information, saying he felt “somewhat” vindicated in his wiretapping claim against former President Barack Obama — debunked by the F.B.I. director and the director of the National Security Agency, as well as the heads of the Senate and House investigations, including Mr. Nunes.