To my mind the only question is residency. If the Santorums meet it, then the district should belt up. If they don't, the district is in the right.
If Santorum were not a legal resident of PA, he would not be eligible to be a US Senator. Tom Daschle actually started the paper work to count his $2 million Washington DC home as his main residence to save on taxes last summer. Astute conservative groups were threatening to file a lawsuit arguing Daschle was no longer a resident of SD, so he was no longer eligible to be a senator from SD. What Santorum really should do is file a motion in federal court.