Of all the arguments against Donald Trump, the softest has been his poor prospects for victory in the general election. True, he has consistently polled worse against Hillary Clinton than have Ted Cruz, John Kasich, and virtually every other person who ran. But polls change. And if Trump were to win the Republican nomination, it would be such an earthquake that the political order might be transformed. Clinton, moreover, has huge vulnerabilities. She lacks many basic political skills, and she is a hostage to fortune—in both the usual way, as the heir to an incumbent president, and in the unusual...