Worrying too much about debates

Based on his performance in last night's slugfest (which I skipped but caught up on this morning), Josh Marshall worries about Barack Obama's ability to defend himself in the general election:

One observation stands out to me from this debate. Hillary can be relentless and like a sledgehammer delivering tendentious but probably effective attacks. But whatever you think of those attacks, Obama isn't very good at defending himself. And that's hard for me to ignore when thinking of him as a general election candidate.

Marshall shouldn't worry too much. There's a general tendency among pundits and reporters to overthink the role of debates, but as the eminent UNC political scientist Jim Stimson shows in Chapter 4 of his book Tides of Consent: How Public Opinion Shapes American Politics, the evidence that they make much difference is limited. By the time the debates happen in the fall, the eventual winner has generally taken the lead that he will hold until the end, as this graphic (which combines the trajectories of 1976, 1980, 1988, 1992, and 2000) illustrates:

Here are the trajectories of the specific races:

While you can tell stories about debates mattering on the margin in a few close elections, Stimson's conclusion is that their influence is vastly exaggerated:

What can we conclude, then, about the debates? What we have seen is perhaps some influence. The evidence is inconclusive to say either that debates matter or they do not. But if they do matter at all, their influence is vastly smaller than, say, the conventions. The reelection landslides show that once voters have decided, debates will not change the outcome.

There is no case in which we can trace a substantial shift to the debates. But in elections that were close at debate times, there are cases (1960, 1980, 2000) where the debates might have been the final nudge. As to why they are so often featured as the central story line of a presidential election campaign, I lean to the idea that they are conveniently available TV footage.

Stimson's argument suggests that Democrats who are concerned about electability should put less weight on debating performance and focus more on other considerations -- in particular, Obama's vastly superior favorability ratings. (Of course, Hillary backers and Ezra Klein will dispute that point, but Jon Chait does a nice job of summing up why they are probably wrong.)