[hga]

They claim to address that, specifically:

Some argue that more liberal candidates are more popular in urban than in the rural precincts. To verify this hypothesis, we drew geographically random samples of precincts and computed partial correlations to filter out the population density factor. We will demonstrate in this paper that this factor has no impact on our conclusions.

I'll have to look at this in more detail when I have time.

Note also that just a few races have to be flipped, "momentum" is really critical. For instance, if they're correct and Romney didn't win New Hampshire, it's very easy to imagine him never getting decisive traction no matter how much money he threw into the campaign (one analysis I recently read noted he campaigned for dollars rather than votes, which of course was telling in the end).