The thing is, if PA didn't have closed primaries Specter would probably win his primary. But indies can't vote in PA primaries, and the PA GOP has been bleeding membership for the past couple of cycles so all that's left are the nuttiest of wingnuts. Basically, if Specter loses that seat is guaranteed to flip because PA at large isn't interested in another Rick Santorum.
Other states liable to flip to blue- OH, FL, MO and now NH where all four have retiring GOP Senators.
Cohen- you forgot the obligatory :libertariansmug