Well, actually, Santorum backed Specter for three reasons:
1) Because Rove and Bush twisted his arm, and said that Toomey couldn’t get elected.
2) Because Bush said that he needed Specter to confirm his SCOTUS choices. Don’t underestimate Specter on that; I don’t trust the guy any further than I can kick him, but he was very skillful at that sort of job. And he succeeded. The Democrats had enough Senators to filibuster, as they have often done to block appointments, but Specter put the pressure on them—before he went back to his usual crazy ways.
3) Santorum endorsed Specter because Specter had endorsed him. It wasn’t easy for a conservative to win in that state, so Specter’s “centrist” endorsement of a conservative was valuable. Rick returned the favor.
Rick later lost in 2006 for a number of reasons. I would chiefly blame Bush and Rove, who won in a landslide in 2004 and then proceeded to blow it by disappointing the base. Few conservatives won in 2006. It was a Democrat landslide. And Rick was further handicapped by a popular opponent and by the stupid Toomey supporters who stayed home in a snit. Good work, guys, putting Casey into office.
The Dems helped one of those numinations along when they where so out there with their attacks that the wife left crying. A case of taking things to far.