We’re in a funny moment. Those who do politics for a living, some of them quite brilliant, are struggling to comprehend the central fact of the Republican primary race, while regular people have already absorbed what has happened and is happening. Journalists and politicos have been sharing schemes for how Marco parlays a victory out of winning nowhere, or Ted roars back, or Kasich has to finish second in Ohio. But in my experience any nonpolitical person on the street, when asked who will win, not only knows but gets a look as if you’re teasing him. Trump, they say.

I...