Democratic Senator Sheldon Whitehouse and Republican Lindsey Graham sent the letter to the FBI on Wednesday.
Mr Trump tweeted the unsubstantiated claim over the weekend.
“Just found out that Obama had my ‘wires tapped’ in Trump Tower just before the victory,” he posted in a series of tweets on Saturday morning.
Despite repeated requests, the White House has not supplied any evidence for the claim.
Mr Graham and Mr Whitehouse, two senior members of the Senate Judiciary Committee, also addressed Wednesday’s letter to the Department of Justice.