He didn't.
I'm one of the last people on this board that you will see pulling the race card, but honestly...it's not a hard pill to swallow when you take everything into consideration.
Our economy is a mess, we're involved in at least one war whose foundation was built on a lie, and the Republican nominee is following a very ineffective administration. You have a Democratic nominee who has bested one of the largest household names in politics (In the primaries) and who, whether you find him inspirational or not, is a great speaker, relatively modest, straightforward, and capable of running one of the most efficient campaigns of our time. He's facing a Republican candidate who is generally considered to be a lackluster persona and poor public speaker. His campaign has stumbled over itself a number of times (Even while the Democratic primaries were shielding him from the attention of those candidates) and he is surrounded by special interests. As with his Democratic counterpart, he's gone back and forth on a number of issues. He supports (And has supported in the past) many of the ill-advised policies of the former administration.
You're right; it's inexcusable to throw a blanket generalization out there and say that everyone who doesn't vote for Obama is a racist (Which nobody here is doing). As I said earlier, I don't know how it can be that the election is still competitive. What I do know is that the people who don't "trust" Obama rarely ever give me a straight answer as to why and I can't help but wonder if they're being evasive because they're too embarrassed to say it out loud.