Democrats on the House Intelligence Committee say they want to move forward with the investigation into Russia's alleged interference in the 2016 presidential election, with or without Rep. Devin Nunes (R-Calif.) as chairman. (Jayne Orenstein,Ashleigh Joplin/The Washington Post)

President Trump has been trying since early March to deflect the FBI and congressional investigations of Russian covert action by arguing that he had been a victim of improper U.S. surveillance. His argument got little traction until last week, when Rep. Devin Nunes (R-Calif.) made the case for him.

Nunes argued that intelligence about Trump associates had been wrongfully collected and shared based on information he had received from an unnamed “source” he met at the Trump White House complex. His charge, and the ensuing uproar, has derailed, at least momentarily, the investigation by the House Intelligence Committee, which he chairs.

Perhaps that was Nunes’s intention — and Trump’s. That’s the upshot of comments by Rep. Adam Schiff (Calif.), the ranking Democrat on the committee, in an interview Wednesday night.

“I don’t know who his source is,” Schiff said. “Here we are a week later, and we still don’t know what information [Nunes] received or who it was from. … We can’t conduct an investigation that way, where we have only the chairman’s word to go on.”

Ever since Trump made his claim on March 4 that he had been wiretapped by President Barack Obama, White House officials have been saying that he would be vindicated. But this allegation of an illegal Obama surveillance order has been rejected, even by Nunes.

Last week, Nunes suggested instead that legal intercepts of foreign intelligence targets had picked up “incidental” information about Trump associates, whose names weren’t properly masked, and that this information was widely disseminated and then leaked. White House officials have privately been sharing this “incidental collection” version since mid-March.

To which Schiff responds: So why didn’t someone bring the evidence to the House Intelligence Committee, rather than leak it to Nunes, who then briefed Trump — bypassing the committee that Nunes chairs?

“If he [Nunes’s source] was a whistleblower,” argues Schiff, “that person would want to share the information with the whole committee,” which has oversight of issues such as proper dissemination of intelligence information. Instead, he said, the goal of Nunes’s informant at the White House complex “was to share the information with the chairman, so that the president can later say that he was vindicated.”

Schiff argued that by end-running his own committee, Nunes had undermined the independence and credibility of the House investigation. “Some way, he has to remove the cloud. … Otherwise, we’re just going through the motions.”

The rupture of the House committee’s investigation was illustrated by the sudden cancellation of this week’s planned testimony by Sally Yates, former acting attorney general, who had been asked to testify about why she warned the White House in late January about then-national security adviser Michael Flynn. Flynn was asked to resign in February after it became clear that he had misled Vice President Pence about his pre-inauguration conversations with Russian Ambassador Sergey Kislyak.

I asked Schiff what questions he would have posed for Yates, if Nunes had allowed the hearing to go forward. “I would have wanted to ask her: What were the circumstances that led to Flynn’s firing? What were the circumstances that led you to fear Flynn could be jeopardized by what he said to Kislyak? How long did the president know that Flynn had lied to Pence before he did anything about it?”

Those questions need answers before Nunes’s committee. He needs to demonstrate that he’s the chairman of a bipartisan oversight panel trusted with the nation’s secrets, rather than a conduit for information from the Trump White House.