Candidate jilted (again) by the GOP

It remains unknown which party will benefit the most from corporations' and labor unions' ability to spend unlimited amounts of money on political ads, Egan said. Corporations tend to support Republicans, while unions typically back Democrats.

Sestak's seat in the Delaware County-based 7th District, which he wrested from longtime Republican U.S. Rep. Curt Weldon in 2006, will be open this year, assuming that Sestak presses on with his campaign against Democratic U.S. Sen. Arlen Specter. Last week, Pennsylvania Democratic Party Chairman T.J. Rooney called on Sestak to run for re-election in the House, but Sestak's campaign said that he still has his sights set on Specter.

In Gerlach's 6th District, recent history has shown that whichever Republican prevails in the primary is likely to have a tough race in November.

And the 8th District race also could be tight this year, creating a replay of the three-way slugfest in the Philly suburbs four years ago. Republican Mike Fitzpatrick, who lost the seat to now-U.S. Rep. Patrick Murphy in 2006 by less than 1 percentage point, announced Saturday that he wants a rematch this year.

In 2006, Democrats won two of the three seats, but some political observers say that the results could be flipped this year if an anti-incumbent electorate expresses its frustration with President Obama and the Democrat-controlled Congress.

Egan sees the potential for confusion among voters in the 8th District, with a congressman named Patrick and a former congressman named Fitzpatrick. The district covers Bucks County and a section of Montgomery County and Northeast Philadelphia.

"The majority of voters pay very little attention to politics and it's quite possible it would be difficult for many voters to remember who the true incumbent is," he said. "It's an amazing recipe for voter confusion, and generally the ads don't necessarily clarify that for voters."