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 Mrs. Clinton have lost ground? Part of the explanation might be that her post-convention bounce hadn’t actually faded by Labor Day. Another possibility 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 stories 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 appearing presidential, he could cement his current gains and even make additional improvement.