If either one was running uncontested, I would agree. But,
Santorum is not that weak. He has a tough race ahead, but not impossible. This primary fight between two different (pro abortion and pro life) candidates is going to weaken the eventual winner. Santorum could roll over the eventual winner by virtue of cash.
Your prognosis may prove out, considering Santorum will be running in an off year and it will harder getting Democrats to get out and vote. The conservative T folks in PA who are staunchly pro-life should come out in big numbers for Santorum, but Casey might skim some of those votes off the top because he, too, is pro-life.
Since Casey, Sr. was treated so poorly by his party's elite, that party has gotten even more left-wing. Casey, Sr. would hardly recognize his own party, if he were still alive.