Santorium was in a lot of trouble long before Casey got the Democrat nod. It was common knowledge for a long time that Santorium was the most vulnerable incumbent in the election cycle. His popularity ratings were low.
By running a pro-life Democrat with a popular family name, the Democrats just eliminated any chance of a comeback by Santorium.
I think Santorum could have dug out a win, but like you said, having a pro-lifer on the Dem ticket made that impossible.