"I don't think there's any question that that's the case, just in the way he conducts himself and goes to such a low level. I just — I do," Corker, R-Tenn., said of Trump in an interview on Capitol Hill.
Corker added, "The worst of it is going to be the whole debasing of our nation. I think that will be the contribution that hurts our nation most."
After the two engaged in a war of words this morning, Corker said Trump wasn't being truthful.
Asked whether he considered the president a liar, Corker said they don't use that "word in our family" but reiterated Trump is "utterly untruthful."
"You would think he would try to focus on things where there wasn't a witness, but the whole world is a witness to these untruths," Corker said.
As for whether he regrets supporting Trump during the campaign and the early days of his administration, Corker smiled and asked, "What do you think?"
"There were many people. I was one of those that hoped he would rise to the occasion as president and aspire to lead our nation instead of dividing it," Corker said, adding, "[Trump] hasn't risen to the occasion."
Corker earlier this morning stood by his Oct. 8 remarks criticizing the White House as an "adult day care center" and arguing that Trump is putting the United States on a path toward "World War III."
His remarks on "GMA" triggered a Twitter exchange after Trump responded that Corker, who's not running for re-election, "couldn't get elected dog catcher," and Corker then called Trump "an utterly untruthful president."
Corker's office denied that he asked for Trump's endorsement, instead saying that Trump urged Corker to run for re-election.
"When you look at the fact that we've got this issue in North Korea and the president continues to kneecap his diplomatic representative, the secretary of state, and really move him away from successful diplomatic negotiations with China, which is key to this, you're taking us on a path to combat," Corker said on "Good Morning America" today.
He added that when it comes to the diplomatic efforts underway to handle the rising tensions with North Korea, he would like for Trump to "leave it to the professionals for a while."
"The president undermines our secretary of state [and] raises tensions in the area by virtue of the tweets that he sends out," Corker said.
Another negotiation Corker wants Trump to stay out of is the tax debate.
Details of the White House's and congressional Republicans' tax plan are still being hashed out, and the details have been kept mostly under wraps.
"Those details we don't know, but we do know in that general area there are a lot of people that wish us harm," Corker said.
U.S. troops should be in Niger, he said, but he and his fellow GOP senators want to ensure that Congress is "playing the appropriate role" in authorizing military force there.
ABC News' Kelly McCarthy and Alexander Mallin contributed to this report.