The median Republican Congressional district now has a population density 11 times smaller than the median Democratic district.
I didn't think the difference would be that large, but the idea makes sense to me. People who live in crowded cities are going to have a greater appreciation of the importance of infrastructure provided by government, and that infrastructure will have a greater impact in their lives.
If you live in the country, you're going to have a greater sense of self-reliance. You're going to see initiative as much more important than cooperation. You're going to have a different perspective on guns if more deer than people are shot where you live.
When Sarah Palin's acceptance speech scoffed at the idea of Barack Obama being a community organizer, I think it was in some ways a function of being from Wasilla. And when Obama made his comments about small town people clinging to guns and religion definitely comes from a Chicago perspective.
So I guess the rule of thumb is, if you can hear your neighbor, he's probably a Democrat. If you have to drive to your neighbor, he's probably a Republican.
Obviously there are many exceptions (Salt Lake City, Big Sur, Sedona), but the statistical correlation is pretty stunning.

http://washparkprophet.blogspot.com/2011/01/population-density-and-political.html