None of the states hammered where going to go for Romney anyway, so whatever reduction in the overall vote happened didn't change the electoral vote. It did, however, give Obama a chance to look very presidential and people tend to line up behind the president during a crisis.
Heck, since Republicans tend to more of followers then Democrats, you could make the case that more are going to be inclined to vote for a Democrat in a crisis then Democrats who would be inclined to vote for a Republican president during a crisis.
It also helped that Obama and FEMA did a much better job then the disaster that happened after Katrina. Looking presidential is nice, actually being presidential is even better.
Jay