One of the few things Hillary Clinton has going for her is that she’s inevitable. But for how long? She will win the Democratic presidential nomination barring the unlikely event that we still live in a country governed by the rule of law. But will she inevitably win in November? We’ve long said no, and at last there’s a poll to back us up.

Quinnipiac is out with its latest swing-state poll, which “focuses on Florida, Ohio and Pennsylvania because since 1960 no candidate has won the presidential race without taking at least...