A good series of charts by Lee Drutman shows that one of the best predictors of declining Democratic partisan ID between 2008 and 2010 is the number of white people:
I used to hold to the view that the growing non-white share of the electorate would, over time, tip elections to Democrats. I now think the system will remain near equilibrium and what we’ll instead see is white voters growing more Republican as Democrats are more and more seen as the party of non-whites. Mississippi and Arizona, after all, have very large minority votes but they’re hardly hotbeds of liberalism. Instead they’re hotbeds of very conservative white people. This does mean, however, that politics will become even more abstracted away from “the issues” and questions of identity will become even more central.