Casey does seem pretty weak here. Granted, people will go in and vote straight Dem but I see a lot of front lawns with Rendell/Congressional candidate/State House candidate signs on the front lawn with Casey missing. I've only seen one Casey sign on a property in NE Philly. Maybe some people just don't want to be vocal about the Senate race...or they might actually want Santorum.
Some may laugh but I think this following statement might be true - people don't want to tell others that they're voting for Santorum. With Santorum down in every poll and his seemingly "crazy" stances, many might not be enthusiastic about vocally supporting him. People might say one thing to their neighbor or a pollster and vote the other way...
People voting for Rendell in the SEPA will no then pull the lever for Santorum. Lay off the drugs they are affecting your mind.