Politically, counties are becoming more sharply divided. Between 1976 and 2008, the percentage of counties where the Republican or Democratic presidential candidate won by 20 points or more doubled from roughly 25% to nearly 50%. Increasingly, we vote like our neighbors.
Curiously, this tendency doesn't just define us as conservative or liberal. It's finer tuned than that. In the primary election between Obama and Hillary Clinton - two senators with nearly identical voting records - half the voters lived in counties where Obama or Clinton won by landslides. We side with our neighbors on differences large or small.
It seems that politics is like fashion, food and accents: it has a distinctly regional flavor.
Facts above come from Bill Bishop's interesting book The Big Sort, Why the Clustering of Like-Minded Americans is Tearing us Apart.