If President Donald Trump is serious about his claims that Trump Tower was wiretapped by former president Barack Obama, Senator Lindsey Graham, R-South Carolina, wants to be able to prove it.

"All I can say is that the country needs an answer to this. The current President has accused the former President of basically wiretapping his campaign," said Sen. Lindsey Graham during told CNN Wednesday. Graham's comments came a day after he had a private lunch with President Trump.

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On the same day that Graham made these comments to CNN, he joined Sen. Sheldon Whitehouse, D-R.I., in writing a letter to FBI Director James Comey and Acting Deputy Attorney General Dana Boente requesting information to substantiate Trump's assertions if they indeed have merit.

"The president's request was for the intelligence committees to look into this, but it is the Department of Justice’s criminal division that obtains warrants for wiretaps, and oversight of the criminal division lies with the Judiciary Committee’s Subcommittee on Crime and Terrorism," their letter said. "Therefore, we request that the Department of Justice provide us copies of any warrant applications and court orders — redacted as necessary to protect intelligence sources and methods that may be compromised by disclosure, and to protect any ongoing investigations — related to wiretaps of President Trump, the Trump campaign, or Trump Tower."

Graham has long been a staunch critic of Trump. Early in Trump's candidacy, Graham called him a jackass (which prompted Trump to give out Graham's cell phone number). More recently, the senator told the president to stop insisting he actually won the popular vote. Graham also joined Sen. John McCain, R-Ariz., in denouncing Trump's first Muslim travel ban as a "self-inflicted wound in the fight against terrorism."