Keep in mind that the we are a closed primary state, in consequence of which a lot of voters change party ID without changing official party registration unless they have a hot primary to vote in, so registration edge is somewhat of a lagging indicator here.
Susquehanna has a pretty good reputation, although I would be happier with D+8 or D+9. However, there are a LOT of lazy Dems in PA, so we win statewide races quite often during the midterm election years.
The registration edge for Dems in PA is that big?? Very surprising considering their congressional delegation usually tilts Republican and I think the GOP has controlled the legislature for most of the recent past. The state is certainly very purple so it must be that a lot of those registered Dems vote Republican in many elections. So I am hopeful Mitt can eke out a win there. If so he will almost certainly take the presidency.
Democrats in PA aren’t the typical liberals you get further northeast. They are mainly two groups. Blue collar Catholic types, who want very conservative social policy on guns, abortion and marriage, but they want high taxes on the “greedy” rich to be redistributed back to them. The other type are basically libertarians, wealthier, who want low taxes for obvious reasons, would be fine with legal abortion, same-sex marriage, and gun control, but would be fine with government making a bunch of fiscal cuts in general.
This makes a lot of PA Democrats potential swing voters, and it also makes them not particularly enthusiastic, which is why they don’t show up for mid-terms that much. Neither party really speaks to everything they really want.