What if President Donald Trump tries to fire Robert Mueller — and fails? The scenario isn’t far-fetched. Under Department of Justice regulations, the special counsel, Mueller, can be fired only “by the personal action of the attorney general” for “misconduct, dereliction of duty, incapacity, conflict of interest, or for other good cause.”

President Trump, who doesn’t much care for legal technicalities, has ramped up his attacks on the investigation into Russian meddling in the 2016 presidential election and on Mueller himself. We know from The New York Times that Trump has at least twice tried to shut down the probe. The president might yet try to fire Mueller directly; his press secretary, Sarah Huckabee Sanders, said Tuesday that the president “certainly believes he has the power” to do so. Or Trump could try to fire Mueller without rescinding the special counsel regulations. Or Trump and his Department of Justice might fail to follow proper procedure in withdrawing the regulations.

If that happened, Mueller very likely would refuse to budge. That is, he would announce that the president lacked the legal authority to fire him, and insist that he was still in office....