Your basic premise might be off. Gallup showed that CHURCH-GOING Catholics and CHURCH-GOING Born-again Christians were both about 68% Republican-leaning. The distinction between Catholics being more liberal than white Protestants from "born-again" denomination is only from two areas: apostate Catholics tend to persist in identifying themselves by their denomination, and tend to have remained Democrat. Church-going Catholics went Republican as issues such as abortion, homosexuality, and the culture wars came to the fore.
Catholics also tended towards urban areas until recently. This correlates them to liberalism, but the correlation is merely incidental: I don't think Mayor David Dinkins got more than 1% of the church-going, anglo Catholic vote.
Urban areas tend to have the most non-religious and the most minorities. As such, they tend to be very Democratic. Even as Catholics have moved out of cities, they have stayed in the suburbs. That means they ended up in states dominated by central cities, and therefore dominated by liberals: NY, MA, NJ, MD, PA, MI, IL, and CA. You don't need Catholics to see central cities become ridiculously lefty. Look at places like Dallas, Seattle and San Francisco.
"Your basic premise might be off."
Yeah, I know. But the thought is still interesting.