Forgive my ignorance, but, what is the Bradley effect?
The Bradley effect is a theory that basically says black candidates underperformed their polling numbers because people are reluctant to go against a black candidate in a poll and appear racist even when they don't intend to vote for them.
It has been disputed whether this is real, but I believe that it is valid under limited circumstances. One of the suggestions is that it depends on who is polling (white or black pollster asking the questions). I personally don't think it applies to conservatives but only liberals suffering from white-guilt. I also think the Bradley effect is normally not an issue because other issues will, most times, be more of a concern. Many have suggested that the election of Obama disproves the theory or at least shows that it is no longer valid. However, I think you will see the Bradley effect in 2012 because race was and has been a major issue with Obama.
Imagine being a white liberal in 2008. You disliked Bush and had the opportunity to prove you were open-minded by enthusiastically voting for a black candidate. Roll the clock forward to 2012. The economy sucks, your job is hanging in the balance, prices are rising, and your struggling to make your mortgage while everyone else seems to be getting bailed out but you. When the pollster calls or your fellow liberals ask you if your are voting again for Obama will you say he's been a failure? (especially when he's running against a whiter than sour cream Romney?)
I only speak from personal observation with no statistical evidence, but in my office building I had many white liberals who were the most enthusiastic Obama supporters you could imagine. Now, the multiple bumper stickers have been scraped off and have not been replaced by "Obama '12" bumper stickers. If you ask them anything about Obama they tend to change the topic outright or at least not give a straight answer. But, they won't come out and say anything negative about him either. They may not end up voting for Romney, but I don't they will vote for Obama. Pretty much every white person I know who happily voted for Obama in '08 shows no enthusiasm this time around. If they voted for Obama to prove they weren't a racist, will not voting for Obama prove they are, at least subconsciously, a racist? I think that is what is going on in their heads. I fully expect the polls to say it is a horse race right up to the end only to see Obama under-perform his poll numbers by a significant margin.