That could be devastating news for Trump's legislative agenda.
Trump and Corker's relationship hit a new low over the weekend in the wake of the Tennessee Republican saying last week that Secretary of State Rex Tillerson, Defense Secretary James Mattis and White House Chief of Staff John Kelly were helping "separate our country from chaos."
On Sunday, Trump took to Twitter to say Corker wasn't seeking re-election because the President refused to endorse him
(a claim Corker's team argues is false).
On Sunday, Corker took things a step further, tweeting "it's a shame the White House has become an adult day care center. Someone obviously missed their shift this morning." In a follow-up interview with The New York Times, Corker accused Trump of running the White House like "a reality show." He also raised concerns that Trump could be putting the US "on the path to World War III."
Corker -- the chairman of the Senate's foreign relations committee -- isn't just a rank-and-file member. He's considered a key leader in the Senate, one who by jurisdiction of his committee and force of personality has put himself squarely in the middle the country's national security debate as well as smack dab in the middle of Trump's latest push for tax reform.
Corker's fears aren't unlike those that have been echoed privately by members and their staff on Capitol Hill, but while many of Corker's allies fear retribution from Trump or his allies in the form of primary challenges and Twitter screeds, Corker's unencumbered now after announcing he wouldn't seek a third term in the Senate
"He has space others simply do not right now," one Republican aide said.
But Corker's ability to provide a check on Trump goes beyond rhetoric. As foreign relations chairman, Corker has the power to advance Trump's nominees for State Department posts and ambassadorships. If Tillerson were ever to resign or be fired, it would be Corker who would have the power to advance a new nominee through his committee. In his role, Corker can limit the President's ability to unilaterally act on foreign relations and Corker is expected to play a key role if Trump decides to decertify the Iran nuclear deal
. If the Trump administration hopes to renegotiate pieces of the deal, experts warn having Corker as an ally on Capitol Hill will be invaluable.
"It works better if they are really on the same page," said Ilan Goldenberg, a senior fellow and director of the Middle East Security Program at the Center for a New American Security. "If (Trump) cannot coordinate and work closely with Corker, it takes away a lot of his options."
On tax reform -- Trump's leading legislative agenda item -- Corker will play another important part. In recent weeks, Corker has emerged as an outspoken advocate against any GOP tax plan that adds to the country's long-term debt, a role he shows no signs of abating.
"Unless it reduces deficits and does not add to deficits with reasonable and responsible growth models and unless we can make it permanent, I don't have any interest in it," Corker said recently at a Senate budget committee mark-up about tax reform.
Corker's warning, which he's repeated numerous times, should be a stark sign for Trump. Republicans don't have much room for error on tax reform. Like health care, Republican leaders are using a process known as budget reconciliation to pass tax cuts. The procedural maneuver gives the Senate the chance to pass tax reform with just 51 votes, but that still means Republican leaders need all but two of their members to support the eventual bill. If Corker says "no" (which he hasn't done yet,) that could undermine Trump's agenda in a big way.
For now, there's no evidence that other Republicans in the Senate will follow Corker in standing up against Trump. Most GOP Senate aides either declined to comment
altogether on Trump's latest feud or simply reiterated that most members won't be echoing Corker any time soon.
"I think anything that keeps us from keeping our promise to the American people is a distraction, but to date most of failing to keep our promises falls at the feet of the Senate, not the President," leading conservative Mark Meadows, the chairman of the House Freedom Caucus, told CNN.
The fact is, Corker may only be one senator, but he may have been the wrong one for Trump to mess with. One person close to the President acknowledged there were concerns about Trump's feud.
"Corker has a significant position of influence and he has friends," this person said, referring to Corker's fellow senators, and adding that any battle with a sitting GOP senator threatens to jeopardize Trump's agenda.