How Much Will the Debate Shift the Polls? Here’s One Calculation.

by John Sides on October 4, 2012 · 12 comments

in Campaigns and elections

In my snap predictions last night, I was wrong about one thing: the post-debate insta-polls definitely suggest that most debate viewers believed Romney won: CNN had it 67-25.  A second poll of uncommitted voters conducted by CBS and Knowledge Networks scored in 46-22 for Romney.  (My second prediction—that the media would reach the same conclusion—turned out better.) One caveat is that insta-poll results don’t necessarily correlate with debate bumps in the polls, as Harry Enten shows.  But still: these results obviously aren’t what Obama wanted.

My ultimate prediction was that Obama’s lead in the polls—now about four points—would narrow by about 1 point.  Does that still seem reasonable?  Here’s another take.  The political scientist Joe Cera looked at the first debate in 2000—one that appears to have moved the polls, as I noted in my piece.  Using a large panel of respondents interviewed before and after the debate, he shows that Bush gained about 3 points, all of which were taken from the pool of undecided voters.  The undecideds shrank from 12% to 9%. 2000 is a useful comparison because roughly similar fractions of people thought Bush had won—57%—as thought Romney won (57% is about what you get averaging the CBS and CNN polls above—with the caveat that they were sampling from different populations).

So let’s assume that the same fraction of undecided voters shift in Romney’s direction as shifted in Bush’s direction.  In 2000, that was 25% of undecideds (3/12=.25).  In 2012, the proportion of undecideds is about 5%.  So 25% of 5% is 1.25 points—about what I predicted last night.  It could end up being a bit more, if you think that Obama has been out-performing the fundamentals of the race, and thus we were due for a course correction.

But if that estimate proves correct—or at least close to it—then as Drew Linzer points out, that’s not enough.  Romney needs to run the table in the subsequent debates, and enjoy favorable news coverage besides (i.e., no more 47% videos).  I tend to doubt that Romney can win the other debates by this same margin.  One of the reasons that the candidates typically fight to a draw in the debates as a whole is that they can often rebound from bad performances with better performances—e.g., Reagan in the second debate of 1984.  We will see.

(NOTE: The post initially described the CBS poll as a poll of debate viewers, which it was not.)

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