WASHINGTON — The Republican chairman of the House Intelligence Committee on Tuesday refused to step aside from the probe of possible Russian ties to the Trump campaign as he won the backing of House Speaker Paul Ryan despite Democrats’ calls to recuse himself.

Rep. Devin Nunes (R-Calif.) said he will continue to lead the investigation even though several committee Democrats questioned his credibility as a close ally of President Donald Trump and his announcement last week he had seen U.S. surveillance of Trump aides.

“Why would I not?” Nunes told reporters, rebuffing Democrats. “That sounds like their problem. My colleagues are perfectly fine.”

Ryan gave support to Nunes with a terse response at a news conference. Asked whether Nunes should step aside and if he knew the source of the surveillance, Ryan said, “No and no.”

Nunes has come under increasing fire after he said Monday that he met a source on White House grounds where he was shown in a secure setting intelligence documents, which last week he cited in a news conference as some support for Trump’s claim that the Obama administration surveilled him.

Nunes acted without consulting Rep. Adam Schiff (D-Calif.), the ranking Democrat on the Intelligence Committee, and did not tell Schiff about the information until he made the announcement to reporters and went to the White House to tell Trump.

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And on Friday, Nunes cancelled all of this week’s scheduled committee meetings, including a public hearing for the testimony of former Obama administration intelligence director James Clapper, former CIA director John Brennan and former acting Attorney General Sally Yates.

Democrats and some Republicans criticized Nunes.

“We are aware that former AG Yates intended to speak on these matters, and sought permission to testify from the White House,” Schiff said Tuesday in a statement.

“Whether the White House’s desire to avoid a public claim of executive privilege to keep her from providing the full truth on what happened contributed to the decision to cancel today’s hearing, we do not know.”

Nunes’ committee spokesman, Jack Langer, said in an email, “The hearing we’d been trying to organize with Yates, Clapper, and Brennan was postponed because, before we speak to them, the Committee wants to bring [FBI] Director [James] Comey and [NSA Director] Adm. [Mike] Rogers back in for a classified session to get answers to questions that they could not answer in our open hearing of March 20.”

Sen. Lindsey Graham (R-S.C.) and Sen. John McCain (R-Ariz.) questioned Nunes’ credibility and the House Intelligence Committee’s ability to conduct its investigation. Graham said Nunes’ story was “a little bizarre” and compared it to an “Inspector Clouseau investigation.” But neither called for him to recuse himself.

Meanwhile, the Washington Post reported Tuesday that the Trump administration last week had attempted to block Yates from testifying, citing letters between her lawyer and Justice Department officials, a claim White House spokesman Sean Spicer called “100 percent false.”

The letters on Thursday and Friday outline claims of confidentiality as a Justice official and executive privilege for communications with the White House.

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But Spicer said the White House never invoked executive privilege and said, “I hope she does testify.”

Rep. Peter King (R-Seaford), a member of the Intelligence Committee, said he supports Nunes and defended his trip to the White House grounds and conveyance of the information to Trump.

Meanwhile, the Senate Intelligence Committee, which is conducting its own probe into the Russian meddling in last year’s U.S. election, will meet in a closed session and its two leaders, Sen. Richard Burr (R-N.C.) and Sen. Mark Warner (D-Va.), will give the public an update.

The Senate committee also announced Monday it will hear testimony from Jared Kushner, a senior advisor and the son-in-law of Trump, about his meeting during the transition with the top executive of a Russian bank on which the U.S. had slapped sanctions.

No date has been set for the testimony.