Why should this surprise Protestants? If the Catholic Church is truly God's church, and its clear Santorum believes it is, then all Protestants are by definition heretics and apostates having abandoned God's church to build up false churches. And heretics can't be Christians even if they use Christian language. It should be obvious that Catholicism assumes Protestants aren't Christians. Of course to be fair Protestants say the same about Catholics. After all they speak of "Christians (themselves) and Catholics" as two different things. So people in glass houses shouldn't throw stones.
True, but Santorum hasn't quite been advertising his Catholicism, now, has he? In fact, he's been doing his best to conceal it.
The way he's been packaging himself to evangelicals, you'd think he was Billy Graham...or Mike Huckabee circa 2008.
I wouldn't be surprised if 75% of Santorum's evangelical supporters thought he was a Baptist. He's truly shameless.
Having watched the clip, though, it's nothing I haven't said myself -- from a different perspective, of course. Clearly, he's talking to a Catholic audience, and the comment is meant to denigrate Protestantism, and to compare Catholicism favorably to it; this may be what bites him in the ass. Although, from a perspective of social conservatism (e.g., positions on divorce, abortion, and homosexuality), the Catholic church IS far more conservative than mainline Episcopal, Lutheran, Methodist, and Presbyterian denominations; gotta give him that.