If this theory is right, Mrs. Clinton is likelier to retake a more comfortable lead than to fall behind Mr. Trump. One could even spin Mrs. Clinton’s narrow lead as good news: She’s still ahead despite some episodes that might have hurt her — like the “basket of deplorables” comment and her pneumonia (and secrecy about it).

The other theory holds that Mrs. Clinton really has lost ground, and that there’s no reason to suppose that she’s likelier to rebound than to keep falling.

Why would she have lost ground? Part of the explanation might be that her post-convention bounce hadn’t actually faded by Labor Day. Another possible reason is that Mr. Trump has been fairly well behaved over the last month. He hasn’t gotten himself into too much trouble, relatively speaking, since his feud with the family of Capt. Humayun Khan, allowing other stories — including ones about Mrs. Clinton — to supplant the steady stream of news coverage about Mr. Trump’s remarks.

If this theory is right, Mrs. Clinton would still be the favorite — she leads today, after all — but she would be in danger. One could argue that she led by a large margin only because Mr. Trump failed to clear even the lowest bars of competence and decorum for a presidential nominee. It would also suggest that the debates are a big opportunity for Mr. Trump: By simply being perceived as acting presidential, he could cement his current gains and even make additional improvement.

Which theory is right? Historically, it’s not obvious that either interpretation is the better one. Sometimes, races really do change for good. Other times, shifts in the polls are just noise.

One thing that helps Mrs. Clinton is that the current pool of undecided and minor-party voters is quite diverse. It includes disaffected young idealists, blue-collar Democrats, well-educated Republican-leaners and probably some of the voters who don’t pay much attention to the news and who just can’t make up their mind between two unpopular candidates. In general, a more diverse pool of undecided voters helps the candidate in the lead, since it makes them less likely to break one way.