Bob Corker. Screenshot/MSNBC Bob Corker couldn't get himself to say it.

During a Tuesday interview on MSNBC's "Morning Joe," Corker, a Republican Tennessee senator who is a top-ranking Trump supporter and has been listed as a potential running mate, refused to answer on three occasions whether the Manhattan billionaire was fit to be president.

"So Mika, here's what I think semiprudent people do — they don't answer questions of conjecture," the chair of the Senate Foreign Relations Committee said when host Mika Brzezinski pressed him a third time on whether Trump is fit for office. "Look, if we get to a point where it seems the pivot is not capable, that there is really no way that this is going to evolve, ask me at that time. Until that time, what mistakes have been made, everyone's hair is on fire, these kind of things happen."

When first asked by the MSNBC host, Corker said it was up to Trump to "take advantage" of his opportunity that anyone "would relish having."

He added that it was time for Trump to pivot from the primary and become a general-election candidate.

"There are armies of people who could help with Treasury functions, with foreign policy, with commerce, that would be willing to help a candidate that, let's face it, hasn't fully formed views on numbers of topics," Corker said. "I mean, part of what people like me do ... is encourage in a direction, and this is an opportunity."

Corker urged Trump "to reassess, to realize where he is."

"This is a very important time for him, it is a very important time for our nation, and you know, it is up to him and I'm going to encourage him with every cell in my body until I see maybe he doesn't take advantage of this," he said when asked by Brzezinski about Trump's fitness for the presidency a second time.

Donald Trump. AP Photo/Jae C. Hong

The senator, who recently visited Trump at his Trump Tower home in midtown Manhattan, condemned the attacks Trump has made about US District Court Judge Gonzalo Curiel. Trump has attacked Curiel, saying the judge cannot be impartial in a case he's presiding over regarding the real-estate magnate's now-defunct Trump University because of his Mexican heritage.

Republicans from all sides of the party have criticized Trump for his remarks, but Trump has refused to change his tone.

Corker first addressed the attacks during a Sunday interview with ABC's George Stephanopoulos on "This Week" and said he did not condone them, but he would not say anything further when pressed.

"Look, the comments that he's made are totally inappropriate," Corker said. "You've well documented that — I've documented that."

"He has no doubt missed an incredible opportunity," Corker later said of Trump missing out on the general-election pivot while Democratic frontrunner Hillary Clinton was still locked in a primary race with Sen. Bernie Sanders of Vermont. "He still has time to pivot, he does, time is running short, but he has time to do that — there's still five months or so until the general election ends, time as he moves toward the convention. He has time."

"I'm going to continue to encourage until it's too late," he added.

Watch parts of Corker's "Morning Joe" interview below: