LAST weekend, for the first time in the ten-month special-counsel investigation into connections between Donald Trump’s presidential campaign and Russia, Mr Trump named Robert Mueller, the former FBI director leading the investigation, in a series of tweets. The missives were not friendly. “The Mueller probe should never have been started,” Mr Trump wrote, as “there was no collusion and there was no crime.” Mr Mueller’s inquiry is “a total WITCH HUNT with massive conflicts of interest!” Days later the president again pressed this criticism, citing the opposition of Alan Dershowitz, a Harvard law professor, to Mr Mueller’s investigation. Concerned that Mr Trump’s tweets might portend another presidential firing, some Senate Republicans—including Jeff Flake and Lindsey Graham—warned Mr Trump against sacking Mr Mueller. (In a moment of institutional grovelling Mr Flake tweeted, “We are begging” the president not to “create a constitutional crisis.”) But if the president ignores these pleas, could he actually fire the special counsel?

Not so easily. Unlike recent flick-of-the-wrist firings of Andrew McCabe, the erstwhile deputy FBI director, and Rex Tillerson, the secretary of state, the sacking of Mr Mueller would be more fraught. Most critically, Mr Trump could not do the deed by himself. By law, the special counsel is appointed by the attorney general—or, in this case, by his deputy, since Jeff Sessions recused himself from the Russia investigation after his meetings with a Russian official during the campaign (which he failed to disclose during his Senate confirmation hearings) came to light.Barring an extraordinary reconceptualisation of presidential authority that could spur a lawsuit by members of Congress, only Rod Rosenstein, the deputy attorney general, may fire Mr Mueller. And the reason cannot be as capricious as a firing on “The Apprentice”. The Department of Justice (DoJ) regulation provides that only “misconduct, dereliction of duty, incapacity, conflict of interest” or another “good cause” can justify removing a special counsel from an investigation.

There is no real evidence Mr Mueller has crossed such a line (despite some theories Mr Trump’s team has floated), and Mr Rosenstein has pledged not to remove the special counsel without cause. But if Mr Trump were to instruct Mr Rosenstein to fire Mr Mueller on the basis of a hunch of impropriety, and the deputy attorney general refused, the constitutional crisis of which Mr Flake warned would probably ensue. The president would probably fire Mr Rosenstein, a move that he seems to have toyed with in recent months. He could then appoint a new deputy or elevate Jesse Panuccio, who has been acting associate attorney general since Rachel Brand left the position in February. Should the new deputy attorney general also rebuff the president and be fired for his troubles, the cycle of sackings and new appointments could continue until somebody removes Mr Mueller as special counsel. Alternatively, Mr Trump could sack Mr Sessions and name a new attorney general without a conflict of interest who could do the job—assuming the nominee could clear the Senate.

There is a precedent for serial sackings leading to the removal of a president’s nemesis, but the story is not reassuring for Mr Trump. In the so-called Saturday Night Massacre of 1973, Richard Nixon fired two justice-department officials before a third (Robert Bork, the solicitor general) finally agreed to write the letter firing Archibald Cox, the special prosecutor investigating the Watergate scandal. But just as Bork then appointed another special prosecutor to take Cox’s place, so any justice-department official who agrees to remove Mr Mueller would have to name another to keep the Russia investigation going. What Mr Trump sees as a witch hunt, then, would probably not end with the demise of Mr Mueller. The inquiry would take a new—and, given the suspicion a Mueller massacre would engender, perhaps more dangerous—turn for the president. Impeachment proceedings against Nixon began four months after Cox’s sacking. By August 1974 Nixon had been forced to resign, lest he be sacked himself.