Minnesota senator Al Franken said the concern of "a few" Republicans arises "in the way that we all have this suspicion" that "he lies a lot".
He added: "He says things that aren't true. That's the same as lying, I guess."
Mr Franken cited Mr Trump's groundless claim that he would have won the popular vote in the presidential race if not for 3 million to 5 million immigrants in the country illegally voting for his Democratic opponent Hillary Clinton.
Mr Trump is also said to have told some senators in a private meeting at the White House that he and former Republican senator Kelly Ayotte would have won in New Hampshire if it had not been for voters bussed in from out of state.
Mr Franken said: "You know, that is not the norm for a president of the United States, or, actually, for a human being."
Culled from Irish examiner