The House intelligence committee is asking the Trump administration for evidence that the phones at Trump Tower were tapped during the campaign as its namesake has charged, a request reinforced Sunday by an influential Republican senator who says the president must either come up with the evidence or retract his claim.

“I think the president has one of two choices: either retract or to provide the information that the American people deserve, because, if his predecessor violated the law, President Obama violated the law, we have got a serious issue here, to say the least,” Sen. John McCain said.

President Donald Trump asserted in a tweet last week: “Terrible! Just found out that Obama had my ‘wires tapped’ in Trump Tower just before the victory. Nothing found. This is McCarthyism!” He continued the allegation against former President Barack Obama in other tweets but offered no evidence.

The request for evidence by Monday was made in a letter sent to the Justice Department by the House committee chairman, Rep. Devin Nunes, R-Calif., and the panel’s ranking Democrat, Rep. Adam Schiff, D-Calif., a senior congressional aide said Saturday. The aide wasn’t authorized to discuss the request by name and requested anonymity.

Obama’s director of national intelligence, James Clapper, has said that nothing matching Trump’s claims had taken place, but that has not quelled speculation that Trump’s communications were monitored by the Obama administration. Trump has asked Congress to investigate.

Early this past week, Schiff said the committee would answer the president’s call to investigate the claim. He also said he would ask FBI Director James Comey directly when he appears later this month before the full committee, which is investigating Russian activities during the election.

On Sunday, Schiff said he doubted there was any evidence of wiretapping, but that Comey and others called to testify at the upcoming hearing “would be in a position to have to know.”

“I think on March 20 if not before we’ll be able to put this to rest,” Schiff told George Stephanopoulos on ABC’s “This Week.” ”I don’t think anyone has any question about this, George. The only question is why the president would make up such a thing.”

McCain said Trump could “clear this up in a minute” if he were to call “the director of the CIA, director of national intelligence and say, ‘OK, what happened?'”

The president has an obligation to provide evidence that Obama broke the law or retract his claim, the Arizona Republican said.

“I do believe on issues such as this, accusing a former president of the United States of something which is not only illegal, but just unheard of, that requires corroboration. I’ll let the American people be the judge, but this is serious stuff,” McCain said on CNN’s “State of the Union.”

Kellyanne Conway, a counselor to the president, said Sunday on Fox News Channel’s “MediaBuzz that the House and Senate intelligence committees have agreed to investigate and “we’ll make a comment after those findings are complete.”

Nunes has said that so far he has not seen any evidence to back up Trump’s claim and has suggested the news media were taking the president’s weekend tweets too literally.

“The president is a neophyte to politics — he’s been doing this a little over a year,” Nunes told reporters this past week.

Other lawmakers also have asked for evidence.

Declaring that Congress “must get to the bottom” of Trump’s claim, Sens. Lindsey Graham, R-S.C., and Sheldon Whitehouse, D-R.I., asked Comey and Acting Deputy Attorney General Dana Boente to produce the paper trail created when the Justice Department’s criminal division secures warrants for wiretaps.

(AP)