PA is special case - In 2006, there was a huge populist movement called “Clean Sweep” - (remember the Midnight Payraise!). Lots of republican vs republican races...unheard of in south central PA! Lots of incumbents thrown out of office.
While this movement targeted the state legislature - the populist outrage spilled over to a few congressmen. I always saw Santorum’s rejection as part of the clean sweep effect (and perhaps a touch of “Bush fatigue”.)
Santorum also lost because he had thrown his support to Arlen Specter rather than Pat Toomey in the previous Senate primary.
Santorum was pressured by the Bush administration to do this, but Toomey was a popular candidate; while Specter was disliked.
Many Republicans therefore refused to pull the lever for Santorum or stayed home in 2006 (I was one) in retalliation.
Not sure that the grudge has dissipated, but I hope it has.
It was the Brian Ross - Mark Foley scandal and the Rove great strategy to not fight the Jay Rockefellar attacks on the Iraqi war !