Less than a month ago, President Donald Trump told Fox Business Network it wasn’t “too late” for him to fire FBI Director James Comey. At the time, I argued that while Trump was correct in a narrow, legalistic sense, “for all practical purposes it is almost certainly false—unless the White House believes that mass FBI resignations, or the appointment of a special prosecutor, or impeachment for obstruction, or some combination thereof, would be an improvement on the status quo.”

That thesis is about to be tested in dramatic fashion, because on Tuesday, Trump took the plunge and fired Comey without warning for allegedly mishandling the investigation into Hillary Clinton’s private email server. We are not yet facing a crisis of presidential legitimacy, but we are on the precipice of one that could be as bad as or worse than any in U.S. history.

No FBI director can or should be considered unfireable. FBI directors are just as capable of negligence, ethical breaches, and crimes as other public servants, if not more so. Comey painted so far outside the lines during the presidential election that the notion that he could or should be fired has crossed many minds over the past several months.

But most FBI directors don’t find themselves leading investigations of the presidents who have the authority to fire them, or serve under attorneys general who have recused themselves from those investigations. Jeff Sessions, Trump’s attorney general, reportedly helped to engineer this firing.

Likewise, there is no precedent for a president terminating an FBI director under nakedly false pretenses. Last year, Trump celebrated Comey’s campaign-season intrusions against Clinton as “bigger than Watergate,” and he may owe his presidency to those intrusions. That Trump now believes those intrusions were so improper as to warrant Comey’s scalp fails the most basic test of good faith.