Most good time-series mathematical modeling is validated against past events in order to predict future unknowns. Furthermore, greater weight is given to more recent events when verifying the model. That is usually the smart way to model a problem–except, that is, when the recent event with the greatest weight happens to be an historical outlier. The 2008 presidential election was an outlier. It was the first election since 1952 when there was neither an incumbent president nor a sitting vice president on the ballot. Since it was a contest unencumbered by incumbency, late-breaking undecideds were not predisposed by external factors to break one way or the other. Going into election day, the RCP average showed about a 7-point lead for Obama over McCain and that’s the way it ended up on election day. In other words, late-breakers broke to each side in about the same proportion as the decided portion of the electorate. However, when there is an incumbent on the ballot, it is uncommon for him to get the late-breaking vote.
Normally, undecideds break against the incumbent. Is there any reason to think that won’t be the case in 2012?