Bill Bishop's The Big Sort is primarily about how geographic segregation affects elections. More Democrats are living in overwhelmingly Democratic neighborhoods, and more Republicans are living in overwhelmingly Republican neighborhoods.
So how does the Big Sort relate to race? Most Americans would like to live in racially integrated neighborhoods. The fact that most people live in neighborhoods in which one race predominates is partly due to the fact that "racial groups" differ in what they think is the ideal mix, which makes it hard for a neighborhood to stabilize with a proportionate mix. The result is more racial segregation than most people want.
Race, though, is a crude proxy for beliefs, values, and way of life -- the very things, Bishop argues, that people are looking for when they choose a neighborhood. Moreover, race is becoming cruder as a proxy for way of life all the time. We can anticipate that as more people choose where to live based on their way of life, and their way of life is less constrained by their race, neighborhoods should get more mixed by race and less mixed by values.
African-Americans, at this moment in history, overwhelmingly share similar political values. Black Americans vote Democratic more than 90% of the time. That is a higher rate of Democratic voting than registered Democrats show. In this year's presidential election, it is likely that 95% of African-Americans will vote for Barack Obama. Given the high baseline of black Democratic voting, the fact that Obama is himself a child of the African diaspora probably does not add very many more votes than any Democrat would have gotten. What Obama will add, I think, are many people, especially young people, who would not have voted at all.
So how does race interact with the Big Sort? Bishop argues that people tend more and more to vote with their neighborhoods. This is partly due to the fact that they move to neighborhoods of like-minded people. It is also due to the fact that the neighborhood line on a candidate or an issue -- which in most neighborhoods is getting pretty easy to discern -- shapes how undecided voters choose. Thus, if most African-Americans live in overwhelmingly Democratic neighborhoods, the new voters, reluctant voters, and undecided voters (small though their numbers may be this time) are likely to fall in line with the neighborhood -- just as the Big Sort predicts.