Americans don't like to be condescended to and really dislike being told that not agreeing with or voting for some inexperienced leftist politician with an African father makes them de facto racists. The media hype is also annoying to many average voters. No one wants to be told, in effect, that Obama is 'practically' president already and your vote is just a formality.
I may be mistaken, but despite Bush's low job approval numbers and McCain's less-than-stellar campaign (so far) I think Obama peaked way too soon. The backlash is growing and I believe he'll go down in flames (metaphorically) in the November election. If prayer can effect that outcome, he must certainly will.
(see also Michigan and Florida primary voters)