You’re pointing to 6 Presidential elections in the last 56 years, and there’s a lot more to it than Presidential elections. In Congressional elections Catholics go for Democrats or moderate Republicans. The excitement for Rick Santorum wasn’t from Catholics, but evangelicals. Democrats weren’t at all nervous about him getting the nomination and drawing away Catholics from their party.
There are no Catholics down south to speak of and that’s why Santorum did well with protestants in the south. The Catholics in the northeast are liberals. He never stood a chance with them. He’s too conservative.