Over the last month, President Donald Trump has attacked kneeling football players, the mayor of San Juan, Senator John McCain, Congresswoman Frederica Wilson, Senator Chuck Schumer, CNN, NBC, ABC, CBS, Hillary Clinton, N.F.L. Commissioner Roger Goodell, Democrats, ESPN host Jemele Hill, Congress, and a grieving war widow. All of these feuds, be they single tweets or multi-week tirades, have collectively lowered the bar—which is already approximately 30,000 feet below sea level—when it comes to expectations around the president's sense of decorum. But perhaps none has been as potentially self-defeating as Trump’s ongoing war with Senator Bob Corker, which is threatening to derail the G.O.P.’s lifelong dream of delivering tax cuts to the rich, as well as Trump’s dream of being able to put a single legislative accomplishment on the scoreboard as his first year in office comes to a close.

To recap, Trump and Corker’s feud began several weeks ago, when the president claimed on Twitter that the senator from Tennessee had decided not to run for re-election because he had failed to secure Trump’s endorsement and realized he couldn’t win without it. In an interview with The New York Times, Corker said that all of these claims were flat-out lies, that the man with the nuclear codes is a dangerous idiot whose impulsiveness could set the U.S. “on the path to World War III,” and that Corker “know[s] for a fact that every single day at the White House, it‘s a situation of trying to contain him,” with the West Wing more or less having been transformed into “an adult day-care center.” Trump responded by claiming the Times had “set Liddle’ Bob Corker up by recording his conversation”—in fact, Corker had requested that reporter Jonathan Martin tape the interview for posterity—and was “made to sound a fool.” On Tuesday, the squabble ramped up when, in a series of interviews, Corker predicted that “the debasement of our nation will be what [Trump] will be remembered most for,” and that the White House should “step aside” on tax reform and let Congress handle it. Trump, of course, responded by attacking Corker in yet another tweetstorm:

Obviously, the fighting could not come at a worse time for Team Trump, which is trying to get a tax bill passed by the end of the year, a goal that would be difficult for even the most qualified, on-top-of-things administration. If Trump could muster an iota of impulse control, he would let Corker‘s (completely reasonable) criticisms go, seeing as he cannot afford to lose the support of more than two Republican senators if he wants to pass tax reform through reconciliation. But of course, he can’t—not even to save his own self-serving tax plan.

Worse, though predictable, has been the response of Corker’s colleagues, many of whom agree with his assessment of the president, but would rather he shut the hell up and pass their tax cuts. On Tuesday, Senate Majority Leader Mitch McConnell told reporters, “I don’t have any observation about” Corker’s comments, while House Speaker Paul Ryan told people to “forget about“ “this Twitter dispute” and focus on tax reform.

As Vox’s Ezra Klein notes, “The widening rift between Corker and the president has put Corker’s colleagues in a difficult position. On the one hand, they overwhelmingly agree with Corker. On the other hand, their entire strategy has been to take their doubts about Trump’s leadership and character and lock them away in the deepest, darkest part of their psyches, to be exhumed once Obamacare is repealed, tax reform is passed, and the legacy-resuscitating memoir is ready to be written.” Sure, they also think Trump is a high-risk lunatic, but is that really worth mentioning when tax cuts are on the line? Meanwhile, the unhinged president on whom they’ve staked their reputations reportedly spent the majority of his big meeting on Capitol Hill complaining about things that his administration “hasn’t gotten nearly enough credit for.”