Bartels' forecast

In an e-mail to Ezra Klein, Larry Bartels adds to the chorus of economic models and polling forecasts that predict Obama winning the popular vote by around 6 points. Then he adds some interesting caveats:

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.

We're all groping around in the dark to some extent on this. Social scientists generally like to change one variable at a time. i.e.: What's the effect of a black presidential candidate? Well, let's run 2000 again only make Gore black and see what happens. This year's election has changed about 20 different variables. It's more like, let's run 2000 again, only make Bush more moderate, make Gore black, make Cheney a woman, have the economy collapse, throw in an unpopular war or two, etc. This is no way to run an experiment.