For much of the past six months, I’ve tried to ignore the political ascent of Donald Trump. Too horrible to be true, I assured myself. But the unthinkable has happened, and Mr. Trump is now the likely nominee of the Republican Party. So what might a Trump presidency bring?

To be frank, the most horrifying aspects of Trumpism are not his economic policies. The worst he could do there would be to precipitate a global depression. It’s far scarier to contemplate an erratic, blustering demagogue taking command of the most powerful...