The Arizona Republican has been one of Trump's most vocal GOP Senate critics since his election. In an MSNBC interview, McCain cast doubts on Trump's claim that 3 million to 5 million people voted illegally in the 2016 election, an unproven assertion he continues to press more than two months after the election.
"There's no evidence of that. And I think that those who allege that have to come up with some substantiation of the claim," said McCain, chairman of the Senate Armed Services Committee.
Trump on Wednesday tweeted that he will carry out a "major investigation" into voter fraud, a probe that he said will include voters registered in two states and dead voters still registered. Those cases are not considered fraudulent unless ballots are actually cast under a dead voter's name or by the same voter in more than one state.