As a former RC and now evangelical, i am quite sure it has little to do with the candidates religion, but the quality overall. Although faith does matter a lot, you are not dealing with a monolithic group.
But Santorum’s recent affirmation of the President as being a Christian testifies to his lack of discernment.
So anyone who is Catholic suffers from a lack of “discernment?”
Santorum's answer was THE Christian answer; as a Christian you must accept the word of someone who professes to be a Christian. A higher power will separate the sheep from the goats.