If white Democrats don't support Obama, it may be because of his race. That's OK with me. I'm against him because of his politics, inexperience, and apparent megalomania. But if the people who would otherwise be for him for the same reasons oppose him because of his race, well, that might redound to the benefit of the country.
But if the white Democrats who oppose Obama (I haven't met any) do so because of his race, it only makes sense. The Democratic party is against them because of their race. And it's in the process of kicking them out of the party.
I doubt you'll find many white Democrats 30 to 50 years from now. They'll be gone by then, essentially kicked out. In fact, John Kerry may turn out to have been the last white Democratic nominee for the Presidency. The Democratic party is morphing into the black and hispanic party. The Republicans are becoming the party of everybody else.
Race has not had much effect on the political affiliations of white people in America so far, but I suspect that's been changing and will continue to change. Blacks and others have for generations regarded themselves as groups of people with common political interests. I suspect whites are starting to also.