— -- Sen. Bob Corker is tripling down on his criticism of President Trump as a leader, telling ABC News today that he believes the "utterly untruthful" commander in chief is "debasing" the United States.
"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."
"I don't make comments I haven't thought about," Corker said Tuesday on "Good Morning America."
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.
The senator was an early Trump ally, endorsing him during the presidential campaign. But Corker has since been wary of how Trump is handling the presidency — in particular, his treatment of Secretary of State Rex Tillerson.
"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.
Trump on Twitter knocked down reports Monday that the tax plan the White House and Republican leadership are drafting would cap retirement savings plan tax benefits.
"There will be NO change to your 401(k). This has always been a great and popular middle class tax break that works, and it stays!" he tweeted.
With Trump planning to travel to Capitol Hill today to pitch his tax overhaul to Senate Republicans during their policy lunch, Corker told "Good Morning America," "What I hope is going to happen is the president will leave this effort, if you will, to the tax-writing committees, let them do their work and not begin taking things off the table that ought to be debated in these committees at the proper time."
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.
Asked whether he buys the administration's argument that economic growth under its tax plan will cut the deficit by a trillion dollars, Corker remained hesitant, "We'll have to see. Obviously, we need to look at scoring mechanisms and go through the process."
As the chairman of the Senate Committee on Foreign Relations, Corker also weighed in this morning on the Oct. 4 ambush in Niger. The Pentagon has opened an investigation into the attack amid questions about how the mission went awry led to the deaths of four U.S. service members.
"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.