Posted by Larry Bartels
Historically (since 1948), about 75-80% of the margin in a typical October poll has lasted until Election Day. If that holds true this year, the current best forecast of the popular vote based on the polls is that Sen. Obama will win by about 6 points.
Historically, best guesses of this sort have been off by an average of about 5 points. Part of that, about 1 point, is due to pure sampling error in the polls. The rest, about 4 points, is due to other sources of error -- most importantly, changes in voters' preferences between the poll and Election Day. Compilations of many polls (such as pollster.com) average out most of the pure sampling error in individual polls, so the relevant average error is probably about 4 points.
Statistical theory suggests that if Sen. Obama is ahead by 6 plus or minus 4, his current chance of winning (the popular vote) is a little over 90%. This calculation ignores (at least) three important factors. First, there is a fairly strong tendency for late shifts in preferences to favor the incumbent party when the economy is strong and the out-party when the economy is weak; that makes Sen. Obama's position stronger than it looks in current polls. Second, there is more uncertainty than usual this year about who will actually turn out to vote, suggesting that the polls may be more wrong than usual; however, given the pattern of new registrations and the apparent strength of the two campaigns' voter mobilization efforts, Sen. Obama seems more likely to benefit than to lose support from unexpected turnout. Third, the possibility that undecided white voters may, in the end, be unable to bring themselves to vote for Sen. Obama on racial grounds makes his position somewhat more precarious than it would otherwise be. It is very difficult to estimate the importance of the racial factor (that is, the extent to which racial antipathy is not already reflected in current polls), but my guess is that this is less important than the other two factors, both of which seem likely to work in Sen. Obama's favor.
These comments were also posted today in Ezra Klein's blog on The American Prospect.