The national polls have shown pretty much zero impact from the Bain tax returns issue since June. But in some swing states, there does appear some small movement (these graphs are made with the least smoothing and begin June 1 of this year):
Obama's national lead is around 2 points. In Ohio and Virginia, it's around 4 – 5 points. Here's Iowa:
Florida and Pennsylvania seem unaffected. And even when you maximize these fluctuations, they remain pretty small. But the ability to make big strides for either side seems limited in such polarized times. A shift of a couple of points in the swing states could determine this election, as it did in 2004.