America sorts itself out geographically into blue- and red zones

From William A. Galston & Pietro S. Nivola, "Vote Like Thy Neighbor," NY Times, May 11, 2008 (bold-faced emphasis added):

... during the past two decades, many whites have moved to one group of cities and many blacks to another. Meanwhile, young people have deserted rural and older manufacturing areas for cities like Austin and Portland. Places with higher densities of college graduates attract even more, so that the gap between such communities and less-educated areas widens further. Zones of high education, in turn, produce more innovation and enjoy higher incomes, generating communities dominated by upper-middle-class tastes. Lower-educated regions, by contrast, tend to be more family-oriented and more faithful to traditional authority.

Not surprisingly, this demographic sorting correlates with a widening difference in political preferences. What’s more, according to Bishop and Cushing, once a tipping point is reached, majorities tend to become supermajorities. This is consistent with the findings of recent political science and social psychology: individuals in the minority of their group tend to shift their views toward the majority, while members of the majority become more extreme in their views. In such circumstances, discussions within groups often intensify, rather than moderate, the underlying polarization.