A relatively low-impact and presumably noncontroversial change to our model. Up until now, we have been estimating the national popular vote by taking our projected margin in each state, and multiplying it by turnout figures from 2004.
Henceforth, we will be accounting for population growth, by using the current Voting Eligible Population estimates determined by George Mason University, and assuming that the same proportion of the eligible population will turn out in each state as did in 2004.
This is by no means the fanciest assumption we could make. There seems to be a pretty clear relationship, for instance, between how close a state is, and what turnout winds up being. We'll likely see much better turnout in Indiana this year, for example, since whether or not the state remains a dead-heat in the polling, it will certainly be much closer than it was in 2004.
But whereas those things are somewhat speculative, the basic demographic reality on the ground is not. States like Texas and Arizona have gained substantially in population since 2004, whereas Louisiana has lost it as a result of Hurricane Katrina. It is straightforward to account for these things.
As most of the population growth is concentrated in red states, this change winds up boosting John McCain's projected share of the popular vote by about four-tenths of a percentage point. However, the way the model is designed, it should not really change the projected electoral vote much at all. It does though reinforce the idea that John McCain is more likely to win the popular vote and lose the election rather than the other way around.