With Obama taking a clear lead in the national polls with about a month left in the campaign, the question on most minds is whether McCain will be able to make an October comeback and win the election. The problem that McCain faces is that an increasing share of the electorate is committing to one of the candidates at this point, and recent history indicates that few are likely to change their minds.
This is an obvious point, but If McCain is going to get back in this race, he can do so in one of two ways: (1) he can win over undecideds or (2) he can change the minds of those who are currently planning on voting for Obama.
Let's look at the first point. I created the chart below using the super-cool new flash tool that Pollster.com rolled out last week. This chart shows the Pollster.com trend for undecided respondents. At the end of August, the undecided trend was around 10%. By October 1st, that number dropped to just above 5%.
Presently, Obama holds a 5.6% margin over McCain in the Pollster.com trend. Thus, even if you allocated every undecided voter to McCain, it still wouldn't be enough for him to overtake Obama (though it would certainly make for a very close race).
Of course, it is highly unlikely that all of the undecideds will go for McCain, so what about the second option--changing the minds of Obama voters? It turns out that in recent elections, it has been fairly difficult to change peoples' minds in October. The National Election Study conducts a panel survey of voters for each presidential election; they interview respondents face-to-face (no cell phone only problem here) in September and October and then re-interview them after the election. This allows us to get a sense of how common it is for citizens to change their minds in the last month of a campaign. I pulled out the respondents the NES interviewed in September of 2000 and 2004 and the results are in the table below. The columns are the vote preferences expressed by respondents during the September interviews and the rows are the candidates that they actually reported voting for when they were re-interviewed after the election.
In 2004, 94.5% of those who intended to vote for Kerry in September reported having stuck with their choice after the election, compared to 95.7% of those intending to vote for Bush who actually did so. The percentages of those sticking with their candidate are just slightly lower in 2000, but the overwhelming pattern here is that very few voters seem to change their minds in October. In 2004, 4.2% of Kerry supporters changed their minds in October and voted for Bush. Even if McCain manages to get that many defectors, it would only improve his standing by about 2% in the polls (UPDATE: This would also cost Obama 2% in the polls, thereby trimming 4% off Obama's margin). But that assumes that there won't be any defections away from his candidacy. What actually happened in the past two elections is that what few defectors there were largely canceled each other out.
What is also striking is that even people who aren't firmly committed to a candidate appear to end up voting for that candidate in November. The chart below divides respondents into those who said that they were "strong" supporters of their candidate before the election or "not strong" supporters. In the last two elections, at least 80% of the weaker supporters of a candidate stayed with that candidate on election day.
The patterns among individual voters are also evident when you look at the aggregate trends in recent presidential elections. You can see this if you look at the charts from Mark's "convention bump" post in August. In 1980, 2000, and 2004, there was virtually no movement in the polls during the final month of the campaign. In 1988, Bush added a little to his lead in October and in 1996 Dole gained some modest ground on Clinton, but in neither case did the October gains make a difference in the outcome. In 1992, Bush gained significantly on Clinton in October, but attracting supporters from the third party candidacy of Perot may have accounted for some of those gains. In any event, Bush still fell short.
Despite the fact that McCain is only down by 5-7% nationally, time is running out and a comeback seems like a tall order. In the new era of partisan polarization, major October shifts in the presidential polls are unlikely. There are few undecided voters left to persuade at this point and in recent elections we've seen that few voters change their minds once they have settled on a candidate.