Wow. The Obama campaign must be very worried about the efficacy of Romney's "you didn't build that" push. They just put out this ad:

This is the first time I've seen the president directly on the defensive in his advertising this year. It's a decent ad, but there's always the risk of pulling a Christine O'Donnell-style "I'm not a witch" ad that protests too much. If I had to guess, I'd say that this is about the Obama campaign being extra-cautious about not letting Romney drive the framing at this stage. It could be overkill, or it could be cautiousness, but an ad like this certainly lets the Romney people know they've hit a serious weak spot.