There is one key respect in which Mrs. Clinton’s big lead doesn’t look so durable, at least in historical terms: She holds only around 48 percent of the vote, and has a commanding lead only because Mr. Trump is stuck around 40 percent. In today’s polarized electorate, he can be polling so low only because he hasn’t unified voters who traditionally lean Republican in presidential elections. Astonishingly, several surveys have shown Mr. Trump with less than 70 percent of self-identified Republican voters.

This type of disunity is the basic story behind two of the biggest post-convention comebacks in modern history, Hubert Humphrey’s late surge in 1968 and Gerald Ford’s in 1976. Both candidates had divisive conventions, and they left without fully unified parties. Both trailed by double digits in August and September polls, often with less than 35 percent of the vote.

But in the end, the Republican faithful returned to Mr. Ford, and most Northern Democrats returned to Mr. Humphrey. Mr. Ford lost by just two percentage points, and Mr. Humphrey by less than one point. The elections ended up as two of the three closest presidential contests of the 20th century.

If you’re a fan of Mr. Trump looking for hope and a relevant precedent for a comeback, this is my best comparison. No, Mr. Humphrey and Mr. Ford didn’t win. But clearly they could have.