Nate Silver says Pennsylvania is a “turnout battle” swing state. The number of persuadable voters is small, so whether or not it swings to one party or the other depends on how well campaigns do at turning out their base voters:
Turnout-battle swing states, meanwhile, are those like North Carolina with relatively evenly-matched electoral coalitions but few persuadable voters in the middle. Pennsylvania is a classic example: can Democrats turn out enough voters in Philadelphia and Pittsburgh to overcome the conservative central portion of the state? It’s pretty much the same question in Pennsylvania every cycle and we often get about the same answer. Virginia, whose demographics are similar to North Carolina, is likewise more of a turnout-battle state. That is not to say there are no swing voters there — Virginia has some in its northern suburbs, for instance — but there just aren’t as many as a proportion of the electorate as somewhere like New Hampshire.