(CBS) – On this election day, both parties are struggling for an edge — any edge.

The turnout — not just the size, but where it’s coming from — is the key. In Pennsylvania, a heavy Republican turnout in the west, the center and the northeast must be countered by a large turnout in Philadelphia.

And here are some scary numbers: in 2006, the turnout in Philadelphia was only 42-percent. A key Democratic source says that it would be a surprise to get 35-percent in Philadelphia this year. That kind of turnout would be disastrous for the statewide Democratic candidates for governor and senate.

Turnout will be a question mark in any national election, with issues like the economy overriding issues that affect individual communities. If this election is a referendum on incumbency, it could be a very long and difficult night for incumbents.

Reported by Larry Kane, KYW Newsradio