Ethnic coalitions are inherently unstable. It used to be a sort of natural law that urban Catholics voted Democratic. Then Reagan won them in huge numbers. And–contra those who are saying that the GOP now has to move left–they didn't win by getting more liberal. Rather, the Democrats got more liberal, on crime and bussing, and the white ethnics who felt victimized by these policies fled. The more ethnic groups you have, the more likely it is that you will eventually find the goals of those ethnic groups in direct conflict. And the Democrats sure do have a lot of groups.