Sure, debates historically have not shifted public opinion much, and so this large move may well prove to be an overreaction.

These changing prediction market prices don’t directly reflect who was more persuasive, but rather which candidate most exceeded expectations. Traders were already aware of the relative eloquence of both Mrs. Clinton and Mr. Trump before the debate. This means that the change in a prediction market price tells you how much each candidate overshot or undershot those expectations. And so it would have been possible for Mrs. Clinton to have made her case more eloquently than Mr. Trump, but if traders had already expected it, the markets wouldn’t have moved.

So my interpretation is that Mrs. Clinton did more — she beat expectations about how she would perform. Or alternatively, Mr. Trump did less, significantly undershooting expectations of a more presidential performance, and thus hurting his chances of winning the White House.