The race warfare between Obama and Hillary could play out like this: Hillary wins the nomination but completely alienates black Democrats, thereby dooming her in the general election. Blacks have voted 90% for the Democrats for the past 40 years and have frequently made the Democrat's margin of victory. Blacks don't have to vote for Republicans to let the GOP win in November, they just have to stay home out of disgust for Hillary's perceived "racism".
I've long predicted that the combating interests within the Democratic party would lead to just this sort of fracture. The Democrats thrive on group-identity politics... live by the sword, die by the sword. Maybe Republicans have an opportunity to show blacks they might prefer a party of limited government, economic conservatism, and social values.