this page. It's called Per Square Mile and features those nifty slider maps contrasting population density by county with vote percentage for Romney and for Obama. The difference is more pronounced for Romney than for Obama, but there is certainly evidence that increased urbanization is working against the Republicans. Population density and Romney's vote percentage are practically photo negative images of each other.
Oh yeah, that little deep blue square in North Texas on the Obama Vote Percentage map is Dallas County. I'm kind of proud of that.
Via Andrew Sullivan.