Sen. Lindsey Graham Lindsey Olin GrahamThe Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Senate Democrats' campaign arm announces seven-figure investment to boost Graham challenger Graham: Comey to testify about FBI's Russia probe, Mueller declined invitation MORE (R-S.C.) said on Wednesday that he'll move to subpoena any documents tied to President Trump's claim that he was wiretapped during the campaign if he doesn't hear back from FBI Director James Comey later in the day.

"He hasn't answered that letter. If he doesn't answer it today, I'm going to go to the chairman of the [Judiciary] Committee, Sen. [Chuck] Grassley [R-Iowa], and seek his support to subpoena," Graham said during a Fox News radio interview.

Graham separately told reporters that he hadn't gotten any indication that he would hear back from the FBI by Wednesday.

"The only thing he can be assured of is if they don't answer my letter, the committee as a whole, I think, will subpoena the documents and I will stay on them to disclose whether or not there's a criminal investigation," he added.

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Graham and Whitehouse, who oversee a Judiciary subcommittee, are investigating Trump's claim, which he offered without evidence, that his predecessor wiretapped him.

“Terrible! Just found out that Obama had my ‘wires tapped’ in Trump Tower just before the victory. Nothing found. This is McCarthyism!” Trump tweeted.

Graham added on Wednesday that he has seen "no evidence of collusion," but his panel will continue digging into whether the Obama administration wiretapped the presidential campaign. "I'm going to find out if there was ever a warrant issued against the Trump campaign. Period," he told Fox News radio.

Graham's comments come after he fired a warning shot to the FBI on Tuesday, telling reporters that the bureau was about to "screw up big time" if they didn't respond to his letter.