I can't help but believe that it has something to do with the Republican un-love of Mitt Romney. He is now looking like the front runner, and if he thinks he is having a rough time just getting the nomination, its going to be even harder once the real campaign starts.
I'm not convinced that there's as much enthusiasm for Pres. Obama, as there was 4 years ago. (From my liberal point of view, he has dropped one too many balls.) But poor Mr. Romney is having trouble gaining acceptance from Republicans. And how can you win an election if your own party doesn't want you. Just ask John McCain.