I realize I didn't explain well the difference between having an incumbent versus none. With an incumbent the undecideds always break for the challenger 2-1. With no incumbent the onus is on the opposition to the party in power.
In 2000, undecideds broke 2-1 for Gore but Bush had a large enough lead to survive it. 2008 resembles 2000 in that it's Obama who must close the deal like Bush in 2000. If Obama is really above 50% in the polls he wins even with undecideds breaking for McCain 2-1. If he's below by even a point or two, the undecideds may turn the election for McCain.
