Today's the day.

The Trump administration faces a Monday deadline to back up the president's incendiary claim – offered without proof – that Trump Tower was wiretapped under President Barack Obama's orders during the 2016 election campaign.

Top lawmakers on the House Permanent Select Committee on Intelligence sent a letter to the Justice Department last week, requesting the agency turn over any evidence that might support the accusation, which President Donald Trump made on Twitter on March 4.

Sen. John McCain, the Arizona Republican and chairman of the Armed Services Committee, echoed the committee's call Sunday, telling CNN in an interview Sunday, "The president has one of two choices, either retract or provide the information that the American people deserve."

House Intelligence Committee Chairman Devin Nunes, R-Calif., took questions during a press conference Tuesday. Win McNamee/Getty Images

FBI Director James Comey reportedly asked the Justice Department to rebut Trump's claim hours after the president's string of tweets.

In an interview Monday, White House counselor Kellyanne Conway said she did not have "any evidence" to back up the president's claim.

"The answer is I don't have any evidence and I'm very happy that the House Intelligence Committee are investigating," Conway told Good Morning America.

The day before, in an interview published by The Record newspaper in New Jersey, Conway had suggested that the surveillance may be even broader, stating “you can surveil someone through their phones, certainly through their television sets – any number of ways" – including, she went on, “microwaves that turn into cameras," a line that has since drawn ridicule.

Conway the next morning insisted she was referring to "surveiling generally," adding "I have no evidence, but that's why there's an investigation in Congress.... Of course I don't have any evidence for those allegations."

The statements echoed comments by some top Republican lawmakers. House Speaker Paul Ryan said in a CBS interview aired Sunday that he had not seen anything to suggest that Obama had ordered that Trump Tower be wiretapped.

House Intelligence Committee Chairman Devin Nunes, R-Calif., told reporters last week that the press was taking the president's words too literally.

"The president is a neophyte to politics," the chairman said. "He's been doing this a little over a year. I think a lot of the things he says, I think you guys sometimes take literally."

Nunes co-signed Monday's letter with ranking member Adam Schiff, D-Calif. It is part of a broader promised committee probe into Russian interference in the 2016 election. A copy of the letter was apparently sent to FBI Director Comey.

"The letter requested certain specific pieces of evidence like FISA applications, if they exist, that would indicate surveillance was being conducted on Trump or his associates during the campaign," a congressional aide said, referring to the Foreign Intelligence Surveillance Court, which grants warrants for monitoring foreign agents.