Megan McArdle breaks Democrats into 3 logically exclusive groups

She asks, "Why Isn't this a null set?":

Democrats who believe that the GOP southern strategy is a defining moment that discredits the entire movement
∩
Democrats who think that the Clintons are very deliberately playing up Obama's race in order to drive white voters into Hillary's camp
∩
Democrats who will vote for Hillary Clinton in November once she secures the nomination.

There may be more, but these seem to have significant coverage.