I'm not so sure it was Obama's credibility that was the issue as much as the IN-credibility from the other side. Basing a campaign on statements by Joe The Plumber and having a VP candidate who couldn't conduct an interview without making some breathtakingly dumb statements. I think those were the issues that dampened people's willingness to discuss Obama's experience and credibility as a candidate for POTUS.

I think situations like this have been referred to as "shooting yourself in the foot".