No evidence they disproportionately prefer the challenger
Some people posting over on Nate Silver’s 538 blog keep making the claim that undecided voters usually split 2:1 in favor of the challenger when they get to the polls. One can imagine a good reason for this pattern, namely that the incumbent is much better known than the challenger. By that argument, people who are still undecided in the last days of the campaign probably have a stronger anti-incumbent attitude than a pro-challenger one. When forced to make a decision, the more powerful negative attitude prevails, and the voter opts for the challenger. (There is also some evidence from psychology that negative attitudes may influence behavior more than positive ones. The emphasis on negative campaigning seems to take this view.)
One could, of course, make the counter-argument, that undecideds may be upset with the incumbent but uncomfortable with the challenger. People with that balance of opinion might eventually choose the “devil they know” over the one they know not.
Unfortunately the proponents of the view that the challenger is the ultimate beneficiary of undecideds rarely if ever muster any evidence for this view. To test this hypothesis requires identifying people who were undecided before an election then later asking them how they actually voted. For this you require what is called a “panel” study where the same people are interviewed both before and after an election.
Now, as it happens, the academic political science community has been conducting just this sort of study for decades. Most of the American National Election Studies include a pre-election and post-election wave of interviewing. I used the 2004 dataset since that is the most recent election where we had an incumbent President running for re-election. The trajectory of the 2012 election also seems quite similar to eight years ago. Here are the results:
The table presents the same data in two different ways. The top table includes all 1,211 respondents who completed an interview in the pre-election wave conducted mostly in September and October. I placed all respondents who did not choose Bush, Kerry, or Nader in the pre-election wave into a residual “undecided” category. (The results are unchanged if only pure undecideds, people who said “don’t know” when asked their preference, are considered.)
The top table shows that it was harder to secure a second interview with people who preferred Nader or who were undecided in the first wave. Nearly half those respondents did not complete a post-election interview, compared to about 30% for major party voters. Since all but one of the respondents in the post-election wave reported having voted for President, we might expect people who failed to complete a second interview were also less likely to have voted. So one plausible suspicion about undecided voters is supported by these data, that undecideds are less likely to vote than decideds.
However, there is no support for the notion that undecideds split disproportionately for the challenger. By my definition of undecideds, Bush and Kerry each received 38% of their votes. If we limit the definition of undecided to only people who explicitly said “don’t know” to the pre-election preference question, fifteen such undecided people said they had voted for Kerry versus fourteen who said they had voted for Bush.
I note with amusement that of the fifteen people who said they would vote for Nader in the pre-election wave not one of them report casting a ballot for him at the polls. Kerry got four of their votes; Bush got two. Half of them could not be reinterviewed.
(Technical note: For the budget conscious like me, it is a lovely thing that the ANES studies are freely distributed as SPSS “portable” files which can be read directly by the open-source SPSS clone, PSPP.)