Samantha Melamed makes the key point about this:
But why should he spend in Pennsylvania? New voter ID regulations could do his job for him by depressing turnout among low-income Democrats.
As Nate Silver explains on the blog FiveThirtyEight, Pennsylvania is the type of swing state that has relatively few swing voters: the numbers of Republicans and Democrats are nearly even, but there aren’t too many independents. By his calculation, we’re the 11th-least-swingable state in terms of numbers of independent-minded voters whose votes might hinge on current events, like an improving (or stumbling) economy. So, really, Romney and Obama aren’t likely to win over many new converts by their ads and stump speeches here. It all comes down to getting out the vote.