If only 5% - 10% defect this election, it will make a huge difference in the outcome. Add the same amount for Hispanics, and the same amount for those that voted for Obamagabe last time but have seen the light, and it is a landslide win for Romney.
We still need for a significant portion of the rest to be attracted away in future elections before Democrats have reason to worry in the long run.
I'll believe it when I see it. I doubt Obama is much worried about these blocs today. So far he has no reason to be.