Do presidential debates matter? Here's the political science evidence.

How much do the debates even matter, anyway? The evidence isn’t entirely conclusive, but in my read of it, debates have the potential to make a small but real impact on the race. Polls have often shifted by a few percentage points during debate season, and in a close race, that could really matter. Now, the effect of general election debates has been overhyped by some. There’s little historical evidence that they’ve ever swung polls by more than a few percentage points. General election debates aren’t like primary debates — there are strong partisan loyalties, the vast majority of debate viewers have already made up their minds about who they’re voting for, and few are willing to change their minds because of what happened in one debate. But, in a close race, with a very polarized electorate, a shift of just a few percentage points could matter a great deal. And even if debates don’t swing the presidential outcome, if they help or hurt a presidential candidate by a few percentage points, that could have a domino effect in down-ballot races — such as the battle for the Senate.

If the media judges Trump by extremely low expectations, or if his outrageous conduct is normalized, that could really affect how some viewers understand what happened. Overall, if Hillary Clinton were still leading Trump by 9 percentage points, then she and her supporters could feel confident that the debates would be highly unlikely to change that. A lead of about 3 percentage points is a different story, though.