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Every once in a while, presidential primary campaigns produce moments of almost surreal suspense, where an outcome that was once unimaginable suddenly seems on the verge of happening.

In 1984, for instance, Walter Mondale looked like he had the easiest path to the nomination of any recent candidate, but after a shocking loss to Gary Hart in New Hampshire, he found himself on the brink of being knocked out by a guy who’d been an asterisk in the polls weeks earlier. In 1992, Bill Clinton seemed to have the Democratic nomination in hand, only to suffer a still-inexplicable defeat in Connecticut to Jerry Brown — which set off talk that Clinton’ candidacy would implode and that a new candidate would emerge if he lost the next contest, in New York. And on the...