Politico has an article up about that ever-present question hovering over the Obama campaign: how many votes will he lose because of the racism factor (the same thing, I feel, applies to Clinton with sexism, but let's focus on Obama for now). Steve Benen also contributes to the discussion.
As far as I can see, most of the talk about the impact racism might have on an Obama candidacy is centered around the "hard-core" racist -- that is, those people who simply would not vote for a Black man, no matter what. Maybe I'm too much of an optimist, but that's not really what worries me. I don't think it's a significant part of the population, and I suspect it's concentrated amongst people who weren't voting Democratic anyway, in areas which Democrats weren't going to win anyway.
The bigger issue, I think, is on the more subtle effects of racism -- the one's that don't manifest themselves so dramatically as a categorical refusal to vote for a Black person. Rather, the more damaging element of racism is that how it might warp the perception of voters of all backgrounds, making them less amenable to Obama's overall message. In America, it is extraordinarily difficult to escape being influenced by at least some racist tropes -- at least at the unconscious level. This is true regardless of one's political alignment, class background, or even (ironically enough) race. But because much of how most people evaluate their political choices relies on a complex and volatile mix of policy preference, instinct, gut feeling, and social pressure, these subconscious cues can play an out-sized role.
Imagine a typical liberal Democrat. She agrees with Obama on most issues, but a few things about him make her a bit uncomfortable. She's not a big of Jeremiah Wright, and wonders if Obama has more Black Nationalist sympathies than he's letting on. She worries about the message his admitted youthful drug use sends to her kids. But ultimately, these concerns pale against the amount of political commonality she has with Obama compared to McCain, so she relatively easily makes the decision to vote for the Illinois Senator.
Now, switch to a more "swing" voter. He is not as lined up with Obama on the issues as the rank-and-file Democrat, though he still shares more with him than he does McCain. But it's a closer call, and for him, those nagging gut instincts (Is he really prepared? How do we know he's not a militant? He's friends with too many rap stars for my liking...) play a bigger role. They could push him over that line that divides between a vote for Obama and a vote for McCain.
This man is not a "hard core racist" by any stretch. If a Black candidate ran that was in line with his views, he'd easily vote for him over McCain, just like our liberal Democrat did. But the confluence of racial factors makes it a harder sell -- it makes it more likely that the marginal voter will end up swinging towards the GOP whereas he might vote for a White Democratic candidate substantively identical to Obama. Those little pushes that racism gives in one direction or another -- those are I think the bigger concern in the 21st century; more so than the standard, easily identified, and easily remedied Klansman form.