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.
Trump's administration is also preparing an executive action that could bring back overseas CIA "black site" prisons, where terrorism suspects were previously detained and tortured, according to The New York Times. McCain, a prisoner of war during the Vietnam War and a leading critic of torture, said he is "confident that it would not stand a day in court if they returned to that."
The Senate in 2015 voted to ban the use of "enhanced interrogation techniques" on prisoners.
McCain did vote this week to confirm Mike Pompeo, who left the door open to such methods, as CIA director.
McCain also expressed some concerns about Trump's nominees for top posts, saying Wednesday he is "leaning against" voting to confirm Rep. Mick Mulvaney, R-S.C., as director of the Office of Management and Budget. He initially criticized Trump's nomination of former Exxon Mobil CEO Rex Tillerson as secretary of State before backtracking and saying he would vote to confirm him.