Reagan and Bush could argue that they’d done popular things in their first term that they needed to follow through to the end. Very stupid popular things, in the case of the Iraq War, but this is neither here nor there. What can Obama say that about? Obamacare? Voters would rather he not, thanks. The stimulus? Best not bring it up. As long as voters thought Mitt Romney inauthentic, yet a terrifyingly rabid right-wing ideologue, Obama didn’t need an agenda; he could just run against the terrible things that Romney would do. But over the last few weeks, that hasn’t been working as well.
When you’re described as a blood-sucking vampire, most of the other guy’s campaign is based on that description, and it turns out you’re not, it shouldn’t be surprising that polls swing back toward you.