The problem is that many Catholics now have a real choice between two pro-life candidates, which has been hurting the Democrats across the country (the loss of the Catholic vote because the staunch pro-abortion stance of most Democrats).
Also, Santorium has done some questionable things, such as asking the Capital Police in DC visit his Pennsylvania home because of Security issues. This looks bad to the Pennsylanian populace - Santorium not trusting the police in his own state.
Because of the bias of the media, Republicans have to work much harder.
Because Casey is not a complete idiot... Santorum has a huge bullseye on him by the rabid leftist, they have made unseating him their top priority... Casey is going to be able to get money and clout he could not remotely have gotten had he run against Specter.
Casey though is basically riding on two things, his fathers name.. and trust me he ain't daddy, and a bit of a backlash against Santorum... Its still relatively early, its really going to matter how Santorum plays his race....
Casey for the most part I haven't even seen or heard from on anything other than immigration and he stuck his foot in his mouth on that one.
Still a lot of time between now and NOV... some folks may not be crazy about Santorum, but by election day Casey won't just be this "not santorum" guy.. who he is, will be well defined and the race will be much different.