In PA the Republicans have 3 issues that count for them.
1) Abortion. Casey's pro-life though so this takes this card out of play.
Hey i'll vote Casey over Santorum any day, any time, but I'm most worried about losing the support of Philly suburban pro-choice RINOs. If you've seen the PA results for the 1980s, you'll see without the Philly burbs, we have less of a chance.
The Philly burbs tend to be socially liberal in general. Casey's pro-life stance is his most conservtaive position, other than abortion he is prettymuch moderate on social issues, while Santorum isn't remotley close to moderate. So while the SE pro-choice group may not like Casey's pro-life views the fact that he is not nearly as socially conservative as Santorum, they will vote for Casey. You could put a pro-life Elmur Fudd up there against Santorum & Santorum would still get crushed in the Philly burbs