If blacks comprise 15-17 percent of the total US population, Obama needs a boatload of whites to vote for him. Among Democrats he did it against Hillary, the pre-race shoo-in.
I think he has convinced enough whites (he didn't have to do much but show up to convince black voters) that he's in fact only half black, so not too much of a threat.
I wonder how it might have been different if he was half black but looked ... white ...
