mccain
Sen. John McCain urged President Trump to reveal the source of his allegation that former President Barack Obama had Trump Tower wiretapped. Above, McCain at 53rd Munich Security Conference, Feb. 17, 2017. Michael Dalder/Reuters

President Donald Trump’s tweets accusing former President Barack Obama of wiretapping him in the waning days of the presidential campaign reverberated Sunday, with Republicans, as well as Democrats, calling on the president to reveal the source of his information.

Last Sunday, former Director of National Intelligence James Clapper flatly denied any taps on the president from agencies under his control. Reports indicate Trump’s accusation may have originated with a conspiracy theory spread by the alt-right website Breitbart.

Read: Clapper Unaware Of Any Order For Tap

The Trump White House has been under pressure amid questions on whether his campaign colluded with the Russian effort to meddle in the presidential election. Intelligence agencies have concluded Russia was behind the hacking of the Democratic National Committee’s emails and some of the fake news stories that surfaced on social media.

Sen. John McCain, R-Ariz., said on CNN’s “State of the Union” said Trump should “either retract [his accusation] or provide the information that the American people deserve.”

“I have no reason to believe that the charge is true, but I also believe that the president of the United States could clear this up in a minute,” McCain said.

“If the allegation is left out there, it undermines the confidence the American people have in the entire way that the government does business.”

Read: Did Trump Meet With Russians Too?

Rep. Adam Schiff, D-Calif., the top Democrate on the House Intelligence Committee, told ABC’s “This Week” he doesn’t expect to see any evidence backing up Trump’s claim and warns his penchant for making wild accusations have “real-world consequences.”

“There are one or two possibilities here: Either the president quite deliberately for some reason made up this charge, or perhaps, more disturbing, the president really believes this,” Schiff said.

“If six months from now the president should say that Iran is cheating on the nuclear agreement, if he's making that up, it’s a real problem. If he’s not making [it] up and it’s true, it’s an even bigger problem because the question is: Would people believe him? Would American people believe him? Would people around the world believe him? And that has real-world consequences.”

House Speaker Paul Ryan, in trying to explain why Trump made the accusation, said in a pre-recorded interview on CBS’ “Face the Nation” he thinks the president is “frustrated with this whole thing about Russia. I think he’s frustrated with selective leaks coming from parts of government that malign his campaign.

“Both the House Intelligence Committee and the Senate Intelligence Committee are going through all of this. That’s the proper place for this. And, by the way, we’ve been presented with no evidence that anybody, any American was in collusion with the Russians on meddling with our election.”

Sen. Roy Blunt, R-Mo., on Fox News’ “Sunday Morning Futures” noted the president could go directly to intelligence officials and ask them to corroborate his allegation, obviating the need for congressional investigations.