A lot of the people that like Obama REALLY like him. Almost nobody really likes McCain. Obama is pimping change. McCain just offers more of the same and things are as bad as they've been in quite some time. It might seem like Obama has a pretty good chance. I'm not so sure.
About 30% of potential voters still support Bush and his administration's vision for America unreservedly. Probably 30% would never even consider voting for a President with African heritage. There is a strong overlap in these groups but probably not too much more than 50%. This means that McCain has 45% of the vote pretty much locked up, even if nobody really wants him to be President.
That's a pretty good head start.