I think part of that was the reflexive blue-collar Catholic, Union voters remembering Bob Casey, SR, and thinking his son would be like him. The other part was that many Republicans were angry that Rick Santorum sided with is Senatorial colleague, Arlen Spector, over Arlen's conservative primary challenger, Pat Toomey.
Three other factors (in descending order of importance, IMO):
1) The larger GOP wipeout of 2006.
2) Santorum no longer had pro-life PA Dems to himself, as Casey Sr. was the last of the true Dem pro-lifers and Casey Jr. ran on that mantle, negating the pro-life advantage Santorum had in the previous two elections, and
3) Santorum did make some missteps that the PA media magnified beyond recognition.
That's some pretty long odds, even for an incumbent - and being incumbent GOP in 2006 was no advantage.
>>>>Only thing that bothers me is that in such a heavily Catholic state, Rick got his head handed to him by that barely sentient Democrat lemming Bob Casey.<<<<
>>I think part of that was the reflexive blue-collar Catholic, Union voters remembering Bob Casey, SR, and thinking his son would be like him. The other part was that many Republicans were angry that Rick Santorum sided with is Senatorial colleague, Arlen Spector, over Arlen’s conservative primary challenger, Pat Toomey.<<
All true, and in addition, I believe that Casey ran as pro-life, pro-gun candidate, neutralizing key parts of Santorum’s message.