I actually think Obama is a bad debater. He's good at a stump speech. He might be good in the Town Hall, where he can talk directly to people and answer their questions.
No, the thing is he can be good when on the stage
alone or with supportive whites, because then he's just a black guy saying something, and he can be judged simply on what he says. When he's in a debate,
he's in direct opposition to a white man - and both he and the white man can be seen, and his blackness is evident to white voters, and the white man's relative whiteness. This is all it takes, it doesn't matter what they say, as long as the white man isn't raving or something (though on the South raving might actually play even better).