Donald Trump has often lambasted potential allies from his own party but now a Republican has punched back, alleging that the president's reckless behavior poses a danger to America.
Senator Bob Corker's extraordinary weekend rebuke, in which he tweeted his dismay that "the White House has become an adult day care center," was a shot heard round the Washington world.
Was Corker patriotically sounding the alarm about a president who could put the country "on the path to World War III," as he told Sunday's edition of The New York Times?
Or, having announced that he will not seek re-election next year, is he merely a lame duck lawmaker relishing the opportunity to throw off the shackles of party unity?
Either way, one of the most respected establishment senators -- one from Trump country in Tennessee, no less -- has gone public with grievances many are sharing in private.
Like Trump, Corker is a businessman turned elected official. He is also the chairman of the influential Senate Foreign Relations Committee, a powerful voice on foreign policy.
Corker fired the first shots last week when he said Secretary of State Rex Tillerson, Defense Secretary Jim Mattis and White House Chief of Staff John Kelly "are those people that help separate our country from chaos."
A furious Trump took to Twitter to demean Corker, alleging the senator had "begged" him for a 2018 endorsement and that he "didn't have the guts" to run for re-election.
Corker hit back, telling the Times that he felt Trump posed a sharp risk to US national security, particularly over his comments about North Korea.
"I know for a fact that every single day at the White House, it's a situation of trying to contain him," the 65-year-old said.
"He concerns me," the senator added. "He would have to concern anyone who cares about our nation."
In his nine months in office, Trump has openly criticized several in his party, including senators like John McCain who refused to back his Obamacare repeal effort.
He has also feuded with congressional leaders such as top Republican Senator Mitch McConnell for failing to get the health bill over the line.
The president branded Senator Lindsey Graham "publicity seeking" for criticizing him over his response to racially-charged violence this summer in Charlottesville.
He attacked Senator Jeff Flake as "weak on crime" and border protection.
And Trump chided House Speaker Paul Ryan for failing to reach a deal on raising the debt ceiling, forcing the president negotiate with Democrats.
But few have spoken out so bluntly against the president as has Corker -- and the president's response has confounded some experts.
With Trump poised to decertify the Iran nuclear pact and leave it up to Congress whether to reimpose punitive sanctions, Corker will play a crucial role in what happens next.
"Corker may be the most important partner in that dance. This is so dumb," tweeted Ilan Goldenberg, a former senior State Department official.
Trump also needs Corker, who has served as an ally behind the scenes, if he does not want his legislative agenda derailed in the Senate, where Republicans hold a slim majority.
The party has a tax reform plan that provides substantial tax breaks for the wealthy, but Corker has signalled he will oppose the legislation if it adds to the US deficit.
Corker has 15 months left in his term, and his remarks imply he is prepared to assume the role of Trump's chief critic within the Republican Party during that period.
He also made it clear Sunday that most Senate Republicans share his view about the volatility of the president.
"Look, except for a few people, the vast majority of our caucus understands what we're dealing with here," Corker told the Times.