Sen. Lindsey Graham speaks in Des Moines, Iowa, on September 19, 2015. REUTERS/Brian C. Frank Republican Sen. Lindsay Graham warned the FBI on Tuesday that it would "screw up big time" if it failed to respond to his subcommittee's letter asking for "copies of any warrant applications and court orders" related to any wiretaps of President Donald Trump.

Graham, the chair of the Senate Judiciary Committee's Subcommittee on Crime and Terrorism, sent a letter to FBI Director James Comey and the acting deputy attorney general last week requesting any information they might have about wiretapping that targeted the Trump campaign before the election.

The letter came after Trump's explosive tweets that alleged, without evidence, that President Barack Obama had Trump Tower's "wires tapped" during the presidential campaign. The White House released a statement shortly thereafter "requesting that as part of their investigation into Russian activity, the congressional intelligence committees exercise their oversight authority to determine whether executive branch investigative powers were abused in 2016."

The president does not have the unilateral authority to order a wiretap on suspected foreign spies operating inside the US, experts say. Doing so would be a federal crime that would implicate career civil servants and necessitate a huge cover-up. Rather, law enforcement and intelligence agencies wishing to monitor signal intelligence they deem relevant to an investigation — in this case, Russia's interference in the 2016 election — must obtain a warrant from the Foreign Intelligence Surveillance Court.

While Trump called for the House and Senate intelligence committees to look into the matter, it is the Department of Justice's criminal division that obtains warrants for wiretaps — a division that is overseen by Graham's subcommittee.

Graham and Sen. Sheldon Whitehouse, the subcommittee's Democratic ranking member, wrote earlier this month that while they "take any abuse of wiretapping authorities for political purposes very seriously," they were requesting that the Justice Department "provide us copies of any warrant applications and court orders ... related to wiretaps of President Trump, the Trump Campaign, or Trump Tower."

"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," the senators wrote.

Graham said on Tuesday that his subcommittee would hold a hearing on Wednesday examining "Russian efforts to undermine democracies." He told reporters that if the FBI did not respond to his letter before the hearing, he would "make an announcement and let the FBI know they are about to screw up big time — if they keep running to the intel committee and not answer that letter."

"We have jurisdiction over the FBI," Graham said. "I like Director Comey a lot. But ... all things legal come through the judiciary committee. If they don't honor this request and give us an answer then I would say that we need a joint select committee because the regular order is not working."

A joint select committee is made up of members of both the House and the Senate and is established to investigate broad, complex issues that touch upon several standing committees' jurisdictions.

Reached for comment, Graham's office said the senator has been "pushing for a Select Committee from the start to investigate all issues related to Russia."

"The Judiciary Committee generally has oversight of the FBI," said Graham's communications director, Kevin Bishop. "The Intel Committee does not. ... We sent a letter to the FBI with Sen. Whitehouse and are awaiting their response."

The FBI declined to comment, except to say that "we provide any responses to congressional letters to the lawmakers."