Forget the hype: general election debates are not game-changers, and in fact the history of them from 1960 on is that they rarely make any difference at all to the results in November.
For a comprehensive look at what political scientists have found about the marginal effects of debates, see an excellent look by John Sides in the Washington Monthly — or just look at the chart presented by Thomas Holbrook. As he shows, the largest swing over the entire debate period over the last six cycles was just a three and a half percentage point loss from Al Gore in 2000 — and of course any change over an extended period may be caused by more than just the debates. On the other hand, this also shows the difficulty facing a candidate who is four points behind nationally entering the debates; even if it goes about as well as it can possibly go, it’s going to be very hard for Romney to make up the ground he needs.
Why don’t debates matter? Most voters have already decided by now. Those who haven’t are almost always not very interested in politics, and so they may not even be aware that there are debates going on. Those who are aware tend to hear the debate through their own filters; it’s as if they have their own personal Fox News or MSNBC, making their preferred candidate sound better and ignoring the best lines from the other candidate or simply finding them annoying.
Even attributing what effects may occur to the debates is a bit dicey. What really matters in most cases is post-debate coverage by the broadcast and cable news networks, which is influnced by, and reflects, what’s going on in the rest of the campaign. So Al Gore’s lousy debate season in 2000 may well have been an effect not of his specific actions during the debates, but how the press was already inclined to present anything new that Gore did as a negative..
People — especially reporters — tend to remember the debates, and so after the election, even years down the road, we all tend to attribute effects to the debates which were really either about the fundamentals of the election that year or about the long slog of campaigning over months. But for the most part, the debates just don’t move very many votes. That doesn’t make them worthless (more on that in a later post, but as far as results of the election are concerned, it’s mostly hype.