Real Clear Politics has a nice review of the many nationally significant races across Pennsylvania this year, decreeing it "the most competitive state in the country":
Democratic Gov. Ed Rendell is term-limited and -- seven weeks before the May 18 primary -- the race to replace him is a toss-up; nine of the 19 congressional districts could be competitive; two of the only GOP districts nationwide that Democrats hope to pick up are in Pennsylvania; and the special election to replace the late Rep. John Murtha (D-Pa.) will likely take on national significance as a key indicator for the November elections.
"Pennsylvania has a history of swinging back and forth from Republican to Democrat, Democrat to Republican in statewide races," said Robb Austin, a Pennsylvania political consultant and former state representative.
This has been true in the Senate -- Democrat Harris Wofford won a special election to replace John Heinz in 1991, and was then defeated by conservative Republican Rick Santorum, who was knocked out four years ago by Democrat Robert Casey Jr. -- as well as in gubernatorial elections.
"If you look at the history going back to the '70s, which in politics is pretty recent history," said Austin, "they elected Milton Shapp, an ultra liberal; followed by Republican Dick Thornburgh; followed by Robert Casey, a moderate Democrat; then Tom Ridge, a war hero moderate; then Ed Rendell, a Philadelphia city liberalist."