Now there's a lot of reasons for that and intensely bad press coverage on McCain versus a fawning, bootlicking Obama-loving press corps had a lot to do with it, 2008 was a reverse-Bradley. People voted for the black guy to look cool and non-racist.
Of course this year, that dog won't hunt, so maybe your Bradley effect will apply. Personally I think the media just outright lies and the REAL polls are looking real good for R&R but they're just not telling us.
MSM poll lies + Bradley effect = R squared win.
The biggest difference this time is that in 2008, Obama was an unknown quantity, so a vote for him was a novel thing to do, and many voters said to themselves, "it can't be any worse than what we have now."
In 2012, we know him all too well, and it is MUCH worse than it was before he was elected. So some of those folks who may be SAYING they'll vote for him again, have no intention of actually doing so.