The top Democrat on the House Intelligence Committee says Chairman Devin Nunes (R) of California should step down from an investigation of alleged ties between President Trump's associates and Russian officials after it was revealed that Nunes met secretly with a source last week on the White House grounds.

The committee is conducting an investigation into Russia's interference in the 2016 election and ties between Mr. Trump’s campaign staffers and Russian officials, similar to an investigation launched by the Senate intelligence committee. The difference, Democrats say, is that Mr. Nunes’s leadership injected partisanship into the process.

House Democratic leader Nancy Pelosi and the top Democrat on the intelligence committee, Rep. Adam Schiff of California, called on Nunes to recuse himself from the investigation, saying Nunes’s close ties to the White House would cloud his ability to effectively investigate the allegations.

The calls come after it was revealed that Nunes met an unnamed intelligence official at the White House last week. This official, he indicated, showed him intelligence reports including the names of Trump associates, which had been shared among agencies after being caught up in "incidental" surveillance that was targeted at foreign officials' communications.

To Democrats, Nunes' investigation seems like an effort to protect the Republican party and the Trump administration, rather than conduct an investigation of serious allegations.

"Chairman Nunes' discredited behavior has tarnished that office," Ms. Pelosi said in a statement. "Speaker [Paul] Ryan must insist that Chairman Nunes at least recuse himself from the Trump-Russia investigation immediately. That leadership is long overdue."

Senate Minority Leader Chuck Schumer (D) of New York said Nunes’s actions made it appear he was more “interested in protecting the president and his party" than in getting to the bottom of the allegations.

"I believe 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.

Mr. Ryan was not eager to heed those calls, and a spokeswoman for the speaker said Monday he has "full confidence that Chairman Nunes is conducting a thorough, fair, and credible investigation."

Nunes says he did not coordinate with Mr. Trump’s aides to arrange the meeting, but he also did not disclose it to Democrats on the intelligence committee, an unusual move on a committee that is working together to uncover evidence.

But Nunes says the documents he examined last week had not been provided to Congress and could not be removed from the White House because of their sensitive nature.

"The chairman is extremely concerned by the possible improper unmasking of names of US citizens, and he began looking into this issue even before President Trump tweeted his assertion that Trump Tower had been wiretapped," said Nunes spokesman Jack Langer.

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The Senate intelligence committee, meanwhile, announced Monday that Jared Kushner, Trump’s advisor and son-in-law, has volunteered to be interviewed about having arranged meetings with Russian officials.

This report contains material from the Associated Press and Reuters.