South Carolina Senator Lindsey Graham has said he is ready to subpoena intelligence agencies for evidence relating to President Donald Trump's surprising claim that he was wiretapped at Trump Tower by his predecessor, former President Barack Obama.

"The President has asked Congress to look into whether or not his campaign was wiretapped by the Obama administration. I will take up that challenge," Graham told CNN. "We sent a letter to the Department of Justice and the FBI asked them for any information that they may have used to obtain a warrant.

"The whole purpose of this," Graham added, "is to find out if a warrant was issued directed at the Trump campaign." Asked whether he would subpoena the information if the agencies do not comply with his request, Graham simply answered "Yes."

Earlier on Wednesday (8 March), Graham and the Democratic Senator for Rhode Island, Sheldon Whitehouse, sent a letter to the Acting Deputy Attorney General, Dana Boente, and the FBI, Director James Comey, asking for any evidence of Trump's wiretapping claims.

"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," the letter read.

"As Chairman and Ranking Member of the Senate Judiciary Committee's Subcommittee on Crime and Terrorism, we would take any abuse of wiretapping authorities for political purposes very seriously. We would be equally alarmed to learn that a court found enough evidence of criminal activity or contact with a foreign power to legally authorize a wiretap of President Trump, the Trump Campaign, or Trump Tower."

Former Director of National Intelligence James Clapper told interviewers that he had no knowledge of wiretaps ordered against Trump or his campaign, adding that he would "certainly hope" he would be aware of any such court orders. For his part, Comey has reportedly asked the Department of Justice to publicly refute Trump's accusations, though no such refutation came about.

Former president Obama is said to be livid at the accusation, and the idea that Trump is questioning the integrity of the Office of the President.