The fact that he's not attempting to get positive media exposure through campaigning as well as general voter fatigue. Edward Snowden's leaks and the Obamcare Debacle didn't help either.
I'll go with this. ACA wasn't Obama's Katrina, it was Edward Snowden. Though he intended to be a patriot, he has conducted himself a spy against us. Many probably wonder what a Republican would have done with him. Bush or any possible 2016 Republican with the exception of Rand Paul could have tried to kidnap or assassinate him for all we know. This all makes me believe that Obama has discredited himself equally among the very liberal who will still vote for his supporters and potential successor (or simply become the "missing voters" that allow the Republicans to become the "establishment machine") and the conservative-leaning "median" voters, who voted for him, but voted for Bush and may vote Republican in 2014 and 2016.
Further, with Israel becoming a greater liability and many Middle East dictatorships becoming failed states, it appears that Obama is neither too dovish or too hawkish but whose Government lacks the social skills to come to a reasonable way forward. This works against Democrats but makes people wary of much more hawkish Republicans. Then again, maybe this creates an opening for a "more competent" Democrat the way it almost did for McCain until the double wammy of Palin and the Lehman Brothers.