Reviewing a book by Bill Bishop, Alan Ehrenhalt writes,

there is one simple statistic, rightly seized on by Mr. Bishop, that is difficult to explain away. It is this: In 1976, less than a quarter of the American people lived in so-called “landslide counties” – that is, counties in which the spread between the two major presidential candidates was 20 percentage points or more. By 2004, nearly half of us lived in this kind of politically tilted territory.

Add this to the list of phenomena that can be explained by the Theory of Everything, which is assortative mating. Fewer people are getting married across class lines. This explains rising inequality, stricter sorting of neighborhoods, the high price of college (as the affluent try to send their children to schools attended by other affluent children), and, well, Everything.