Senator Bob Corker said Thursday he doesn’t think President Donald Trump has demonstrated “stability,” “competence,” or understanding of “the character of this nation.”

Normally, that would be a stunningly personal attack for a senator of one party to launch against a president of the other party. But Corker and Trump are both — at least in name — Republicans.

Aside from the most prominent issue at hand — Trump’s reluctance to unequivocally denounce white supremacists in the wake of lethal violence in Charlottesville, Virginia, last week — there’s a lot of bad blood between Trump and his putative Republican allies in the Senate because of his attacks on Majority Leader Mitch McConnell, embattled incumbent Sen. Jeff Flake, R-Ariz., and others.

It shouldn’t make much difference to Trump if he has to fend off a few verbal arrows from senators who want to position themselves against him. After all, Corker’s the chairman of the Foreign Relations Committee, where major constituencies include people with good historical reason to hate Nazis. (Yes, you can read that as “It’s tough to run the Foreign Relations Committee without backing from pro-Israel Jewish groups.”)

With a capital ‘T’

But there’s something deeper going on here that spells trouble for Trump.

On Friday morning, I sent a note to Corker’s office asking whether he would support Trump for renomination in 2020.

“One thing your boss didn’t address to my knowledge is whether he supports President Trump for renomination in 2020,” I wrote. “Since the president’s re-election campaign is up and running, and in light of Sen Corker’s concerns about the president’s stability, will he weigh in on Trump for re-nomination?”

And here’s what I got back from Corker’s communications director, Micah Johnson: “I don’t have anything to add to the senator’s comments yesterday.”

Wait. What?

It could be that Corker comes out in the next few days and says he’s behind Trump 100 percent, but even the hesitation is telling.

It’s barely been nine months since Trump won Corker’s home state with more than 60 percent of the vote. The next election, which Trump started campaigning for upon being sworn in, isn’t for more than three years. It should be a layup for a Republican senator from ruby-red turf to endorse his party’s sitting president for renomination.

I expect a lot of Republican lawmakers are going to be asked in the coming days whether they will endorse Trump for renomination, and the responses will give an early measure of where the president stands after eight months of uninterrupted tumult.

Whether Republicans endorse Trump just shouldn’t be a real question so soon in his first term — if ever. But it is. Trump is in danger of losing the support of anyone who isn’t absolutely convinced that he or she has more to lose by abandoning him than by sticking with him.

Can Trump win again without Republican elected officials on his side? Certainly. But it will be harder if they actively oppose him rather than doing what many of them, including Corker, did in 2016 — give him substantial cover even as they withhold their official endorsement. And, more important for the moment, he doesn’t have a prayer of governing without them.

Corker framed his criticism in terms that allow for the possibility that Trump will, in the future, show that he’s stable, competent and in touch with the nation’s values.

That is, Corker set himself up to flip toward Trump if the president changes his ways. Already, Republican lawmakers have bent over backward — at great peril to their party and their own reputations — to avoid a permanent rift with the president.

How many chances?

But Trump seems to be running out of chances to prove himself, even with loyal Republicans.

They were notably unwilling to defend him in the wake of his stumbling reaction to the Charlottesville violence. Some, including White House aides who spoke to reporters on the condition of anonymity, rushed to distance themselves from the president.

He’ll have yet another opportunity to prove that he’s capable of leading this great nation when he delivers an address in Phoenix next week.

He can’t make things right — those who believe he is a bigot won’t accept a single speech as evidence to the contrary — but he has a chance to make them better. No one has any reason to expect a course reversal. That’s not in Trump’s nature. He’s constitutionally opposed to backing down.

But it would be in the best interests of the nation — and, more important for him, in the best interests of the party and of repairing his relationships with fellow Republicans — if he sounded more like the president of a diverse nation than a tepid politician trying desperately not to alienate a portion of his base that doesn’t deserve a home in either major political party.

If he doesn’t do something to clean up the mess, it will be Republicans, not Democrats, who are most eager to push him toward the political exits.

Roll Call columnist Jonathan Allen is a co-author of “Shattered: Inside Hillary Clinton’s Doomed Campaign” and has covered Congress, the White House and elections over the past 16 years.