U.S. Senators Lindsey Graham (R-S.C.) and Sheldon Whitehouse (D-R.I.) on Wednesday sent a letter to the Department of Justice (DOJ) and FBI asking for evidence to back up President Donald Trump’s allegation that President Barack Obama wiretapped him before the November election.

Trump took to Twitter on Saturday to make the claim, without providing evidence, and on Sunday the White House asked Congress to investigate.

White House press secretary Sean Spicer on Monday also presented no evidence to back up the president’s allegation, but said, “there’s no question that something happened.”

In their letter to Acting Deputy Attorney General Dana J. Boente and FBI Director James Comey, the senators are requesting proof.

Graham is chair and Whitehouse is ranking member of the Senate Judiciary Committee’s Subcommittee on Crime and Terrorism, which, they note, has oversight of the Department of Justice’s criminal division, which obtains warrants for wiretaps.

“Therefore,” they write,

Graham also said to CNN Wednesday that he would subpoena the intelligence agencies for evidence.

Also on Wednesday, Republicans on the House Judiciary Committee, “the primary House committee of jurisdiction over the Foreign Intelligence Surveillance Act (FISA) and the Wiretap Act,” sent a letter (pdf) to Comey calling for a briefing on the wiretap allegation as well as Russia’s alleged interference into the election.

Former Director of National Intelligence James Clapper has denied the wiretapping claim, an Obama spokesperson called it “unequivocally false,” and Comey reportedly urged the DOJ to refute it. 

But, Brain Barrett writes at Wired, “If federal authorities did have cause to listen in on Trump Tower, though, and they provided enough evidence for a FISA court to approve the snooping, Obama is not the one who ought to worry.”

And “if the allegation is not true and is unsupported by evidence, that too should be a scandal on a major scale,” argues Noah Feldman, a professor of constitutional and international law at Harvard University. “This is the kind of accusation that, taken as part of a broader course of conduct, could get the current president impeached.”