On October 1st I noted that we weren't likely to see much movement in the polls in October. This was based on survey data from the past two campaigns that indicated that few voters tend to change their minds once they have settled on a candidate. Of course, if an October comeback was fairly unlikely, then a final week comeback is undoubtedly a longer shot.
According to this site's national trend estimate, Obama's margin is almost twice as large as the percentage of undecided voters left. Thus, at this point, McCain's only path to victory involves attracting support from voters who are currently planning to vote for Obama. This certainly isn't impossible. After all, pre-election polls ask voters which candidate they would vote for if the election were held today. Just because a respondent says they would vote for a particular candidate if the election were held today does not necessarily mean they have made a final decision on that candidate. For example, the ABC News/Washington Post tracking poll indicates that 9% of respondents who have chosen a candidate say that there is a chance they could change their minds. Should this give the McCain campaign any hope?
To answer this question, I looked at exit polls from the last four presidential contests. In each year, the exit polls included a question asking respondents: "When did you finally decide who to vote for in the presidential election?" This question provides a way of capturing which voters had not completely settled on their vote choice until the last week of the campaign. The chart below plots the percentage of late deciders across the past four presidential elections.
Interestingly, voters appear to be settling on their vote choices earlier in recent campaigns. In 1992, one-quarter of the electorate said that they did not come to a final decision until the final week of the campaign and the figure was about 30% in 1996. However, by 2004, nearly nine in ten voters reported that they had settled on their presidential vote choice before the final week of the campaign. Whether it is because of increasingly longer campaigns or heightened polarization, the fact that voters are making their final decisions earlier in recent contests does not bode well for the McCain campaign. In short, there are fewer late deciders to win over than there were in previous years.
Nevertheless, according to the ABC News/Washington Post survey, 8% of Obama supporters say that they could still change their minds (11% of McCain supporters say the same). Is there a chance that McCain can win over those Obama supporters that still have doubts while holding on to his own soft support? Recent history suggests that such an outcome is unlikely. The chart below breaks down the vote choices made by those who said that they came to a final decision during the last week of the campaign.
According to the evidence from the exit polls, in every election since 1992 Democrats have fared better than Republicans among late deciders. Of course, it may be the case that in these elections Democratic support was softer than Republican support going into the final week. But in none of the last four elections did late deciders break more for the Republican candidate than for the Democrat. Once again, this pattern does not give the McCain campaign much hope.
Overall, current polling and recent history suggests that there is little hope for a final week comeback. There are fewer late deciders in recent elections than there have been in the past, a pattern that seems to be holding in this contest. Furthermore, late deciders have tended to break more for the Democratic candidate in previous elections, not the Republican. Thus, the prospect of a McCain comeback seemed quite unlikely at the beginning of October and it appears to be truly improbable now.