Why Did Romney Bounce?

Nate Cohn puts recent polling under the microscope:

Obama's past flirtations with 49 percent of the vote suggest that he might have plenty of latent supporters on the route to 49 or 50 percent. If Obama's losses are the result of marginal voters doubting the president after a mediocre performance and accompanying media coverage, then Obama will probably win many of these voters back over the next few weeks. But the decision of these voters to embrace the president may have been informed by a more negative impression of the Republican nominee than voters will receive over the final month, between strong Romney debate performances and more balanced ad spending. If Obama's losses are more a product of Romney disrupting negative preconceptions than voters doubting the president after a weak performance, then Romney might be able to fight with Obama voter for voter through Election Day.