I think it’s because of a few things:
-As others have rightly pointed out, picking Paul Ryan was one of the best moves Romney has made so far this campaign. Conservatives on the fence about Romney got a major reassurance, and independents are seeing a guy who is experienced, smart, likable, and serious about the issues.
-Obama is out of excuses. He’s four years into a four year term, and people are tired of hearing about what he inherited. They’re tired of bad jobs growth, high fuel prices, and high food prices (something that isn’t get a lot media attention, but matters a lot to families on a budget). Voters are looking for solutions, not empty promises, and Obama has none (solutions, that is).
-Obama/Biden are scaring the heck out of people with their “base rallying” statements, and ticking off people with their increasingly shrill attacks. I’m not going to say negative campaign is ineffective, because history has shown otherwise. But there is a big difference between pummeling someone on the issues and the sort of spew we’re getting out of the Hopey-Changey team these days. I still believe that the overwhelming majority of American voters are, by and large, are a fair-minded bunch. And I believe a lot of those fair-minded folks have a growing disgust for the kinds of things Obama and his team are putting out there.
This is a very salient point. No President has won re-election by blaming those who came before.
Americans do not like a President who won't own what he's done, or what he hasn't done.
If I were R-R, I'd be mentioning that -- a lot.