The Obama campaign's focus on Mitt Romney's time at Bain has been successful in one regard: they have kept the media's attention on that story for a few weeks. But there's no evidence, yet, that it's changed voters' minds. The polls continue to show a very close race, with Obama very slightly in the lead. Romney's ratings have shown no significant deterioration, nationally or (as far as I'm aware) in battleground states, where Obama's attack ads have been running day and night.
The Democrats may argue that such things take time to feed through to voters' perceptions. They may also believe that they are neutralising Romney's business background, turning a potentially winning issue for him into a draw.
There's another possibility, though, more worrying from their point of view: that the net effect of all this attention on Romney's track record is to inform voters who haven't been paying attention of one salient fact about the Republican candidate: he's a businessman. And hey, isn't a business guy just what we need right now?
That might explain why a new poll shows Romney with "a significant edge" over Obama when it comes to managing the economy. Given that economic competence is likely to be the most important factor in determining the identity of November's victor, that is quite alarming for the Democrats.