Republican Sen. Bob Corker runs the powerful Senate Foreign Relations Committee, is widely seen as a moderate and sober voice on national security issues, and has won the respect of both Democrats and Republicans for his handling of thorny issues like Iran and Russia.

He is also now warning that President Donald Trump is setting the US on a potential “path to World War III.”

“He concerns me,” Corker told the New York Times. “He would have to concern anyone who cares about our nation.”

It’s hard to overstate the significance of those comments, or of the rest of the interview (it’s worth reading the entire article by reporters Jonathan Martin and Mark Landler).

Just as importantly, Corker flatly told the Times that he’s not the only Republican with serious concerns about Trump’s character, policies, and temperament.

“Look, except for a few people, the vast majority of our caucus understands what we’re dealing with here,” he told the newspaper, going far beyond what even many of Trump’s sharpest Democratic critics have been willing to say, at least in public. "Of course they understand the volatility that we’re dealing with and the tremendous amount of work that it takes by people around him to keep him in the middle of the road.”

That, in a single quote, may the most jarring thing about his criticism of Trump. Corker is saying out loud what I hear privately from sources throughout the military and the US government, and from both foreign ambassadors and visiting foreign diplomats. The broad consensus is that Trump is a uniquely dangerous commander in chief, one whose inconsistency, dishonesty, immaturity, and inability to control his anger pose a genuine risk to the national security of the US and the stability of the world as a whole.

These fears, whispered for months, have now been brought to the surface by a member of Trump’s own party.

To be sure, there’s a political element to Corker’s criticism. The Tennessee Republican recently announced that he won’t be seeking reelection, freeing him to speak his mind without fear of political repercussions. He’s also been locked in a nasty Twitter war with Trump, who mockingly said Sunday that Corker “didn’t have the guts to run” for another term. Corker shot back that Trump’s White House was an “adult day care center” where someone had “missed their shift.”

But set aside the spectacle of a president and senator of the same party publicly exchanging such nasty and personal attacks, and take a close look at the substance of what Corker has been saying about Trump in recent weeks. A top Republican is now saying out loud what foreign leaders around the world have been saying privately about Trump’s handling of the nuclear standoff with North Korea: It’s been so ham-fisted and bellicose that it could trigger a catastrophic war that kills hundreds of thousands or even millions of people, including tens of thousands of Americans.

Ignore the personal attacks. The substantive ones are much scarier.

Corker has been escalating his attacks on Trump since August, when Trump repeatedly defended some of the neo-Nazis and white supremacists who marched through Charlottesville, Virginia. Trump, Corker said at the time, “has not demonstrated he understands the character of this nation."

The rest of Corker’s interview was overshadowed by the senator’s comments about Charlottesville, but they gave an early sense of his profound concern about Trump’s ability to navigate the nation through crises like the one in North Korea.

"The president has not yet been able to demonstrate the stability nor some of the competence that he needs to demonstrate in order to be successful," Corker said in a video posted by a local news website called Nooga.com.

Absent “radical changes” in the White House, Corker continued, the dangers would only grow.

Trump does seem set to make a radical change to his administration, but in the exact opposite direction of what Corker has been calling for.

There’s a growing expectation across Washington that Secretary of State Rex Tillerson will soon either quit or be fired by Trump because of perceived disloyalty (Tillerson reportedly called Trump a “fucking moron”) and lingering policy disputes. Those disagreements aren’t trivial, and range from the Iran nuclear deal (Tillerson wants to stay in; Trump wants out) and the standoff with North Korea (Tillerson wants diplomacy; Trump thinks that’s a waste of time).

Corker is one of Tillerson’s closest — and only — allies on Capitol Hill, and has been warning for months that ousting the former Exxon Mobil CEO would be an enormous mistake that directly endangers the national security of the US.

Speaking to reporters on Wednesday, Corker said Tillerson, Defense Secretary Jim Mattis, and White House Chief of Staff John Kelly “are those people that help separate our country from chaos."

Tillerson, Corker added, “is in an incredibly frustrating place” and “not being supported in the way I would hope a secretary of state would be supported.”

Corker’s concerns about Tillerson’s well-being may soon become moot; a “Rexit” of the nation’s top diplomat seems inevitable, with the only question being when and how it takes place.

Once it does, we’ll get a chance to see if Corker is right that the biggest problem with the Trump administration — and the thing likeliest to trigger a war no one wants — is Trump himself. That’s been the fear world leaders have been quietly discussing for months. A top senator from Trump’s own party is now saying that they’re right to worry.