Mr. McGahn apparently was able to dissuade Mr. Trump from issuing the order to prosecute political enemies by telling him that the plan was so antithetical to American political values that it could trigger impeachment proceedings. (One sympathizes with Mr. McGahn trying to get his head around the order in the first place and attempting to explain to his boss, with the patience due an errant schoolchild, how profoundly out of line he was.)

Mr. McGahn is hardly a Lincolnesque figure, but he is sufficiently steeped in American legal and political culture, and sufficiently experienced, to be able to rebuff the president. The screaming question here is what could have happened — or might happen in the near future — if the order had been issued to Trump crony Matt Whitaker, now the acting attorney general, through the incoming White House counsel, Pat Cipollone, a far more callow figure than Mr. McGahn. After all, both Mr. Cipollone and Mr. Whitaker were most likely chosen in large part for their moral malleability — a quality Mr. Trump has repeatedly and loudly demanded from his branch’s lawyers.

I hope and believe that the order to prosecute — without even a predicate of criminal activity — would have prompted a raft of resignations at the Department of Justice. But not everyone would have left, and if the attorney general commanded it, it probably would have been put in motion.

Presumably the prosecution would have been halted before Mrs. Clinton was led away in handcuffs. But possibly not, given doctrines of deference to the executive branch, which typically leave defendants scant room to question indictments based on improper prosecutorial intent. In any case even the mere initiation of the prosecution would have been a severe injury to American democracy.

It’s the sort of injury, moreover, that would have been near-inconceivable before 2016. The principle that the president may not simply prosecute political enemies is part of the DNA of American political culture, but Mr. Trump is the heedless radioactive force indifferent to the nature of the bonds he is breaking.