Tom Coburn, a medical doctor who served for 10 years in the House of Representatives upper chamber between 2005 and 2015, said he nonetheless thought that his core voters would continue to back the former reality TV star.
"We have a leader who has a personality disorder," he told The New York Times. "But he’s done what he actually told the people he was going to do, and they’re not going to abandon him."
The latest of a number of senior Republicans to rebuke their leader, Mr Coburn's comments came senator Jeff Flake launched a blistering attack on Mr Trump.
Taking to the Senate floor earlier this week, the Arizona politician said Mr Trump's behaviour was "dangerous to democracy".
After announcing that he would not seek re-election in 2018, he said he had “children and grandchildren to answer to."
"Reckless, outrageous and undignified behaviour has become excused and countenanced as telling it like it is when it is actually just reckless, outrageous and undignified," he said. "And when such behaviour emanates from the top of our government, it is something else. It is dangerous to a democracy."
He added: "It's also clear to me, for the moment, that we have given in or given up on our core principles in favour of a more viscerally satisfying anger and resentment. But anger and resentment are not a governing philosophy."
"He’s obviously not going to rise to the occasion as President," said Mr Corker, the chairman of the Senate foreign relations committee. He added that “the debasement of our nation” would be what Mr Trump “will be remembered most for”.
He added: "I’ve been with him on multiple occasions to create some kind of aspirational approach, if you will, to the way he conducts himself. But I don’t think that that’s possible. Unfortunately, I think world leaders are very aware that much of what he says is untrue.
"I don’t know why he lowers himself to such a low, low standard and debases our country in the way that he does."
Mr Trump fired back in typical style, tweeting: "Corker dropped out of the race in Tennessee when I refused to endorse him, and now is only negative on anything Trump. Look at his record!"