Its priming below your cognitive threshold. Drive in theatres used to splice pictures of cheese burgers between frames of film. You'd think you're watching a movie but half way through you're hungry as fuck.
Certain combinations of words have been studied to have a similar affect (angry + black = isolate). The words are used in a speech to convey one point, but purposely chosen since they sometimes appeal to a different unconscious point as well.
He's not saying we're all racist. He was pointing out that Romney's using a questionable psychology trick. As to its effectiveness, that's also questionable.
Both Republicans and Democrats have been known to use them. The guy's not gonna call out his own party on TV though.