Could be. Someone would need to be a pretty deep-dyed racist to vote against Obama because of his skin colour, but I'm sure there are plenty like that. But I'd also imagine there'd also be quite a lot of people who mightn't be totally sold on him, who might be swayed to vote for him in a general election, as a way of asserting that the USA isn't any longer a racist country. (And the same could apply to Clinton, for analogous reasons, if she turns out to be the Democratic candidate.)

Opinion polls are pretty suspect things, but this round-up of them seems to indicate that either Obama or Clinton would stand a pretty good chance of beating the likely opponents, and there's not a lot between them.