Americans today see two contradictory futures looming in the middle distance: In one scenario, Donald Trump is convicted of serious crimes and sent to prison. In the second, he returns to the presidency in 2025.

The urgent uncertainty of it all may be a reason why, in recent weeks, Trump has summoned forth some of the most incendiary rhetoric ever employed by an American presidential candidate. He has called for the execution of the former chair of the Joint Chiefs of Staff. He now urges police to shoot shoplifters. He has begun characterizing his political adversaries as subhuman “vermin” who must be “rooted out.” Parroting the Third Reich while claiming (no doubt truthfully, in this case) that he has never read “Mein Kampf,” Trump has declared that immigrants from Latin America, Africa and Asia “poison the blood” of the United States.

And yet Trump is cruising to a third consecutive Republican presidential nomination, and many national polls show him beating […]