If one part of the Democratic Party's coalition is built on united opposition to an evangelical Christian conservative ideology, then Catholics have to be a part of that coalition.
Here's more evidence from I came across by way of American Samizdat.
A Charlotte, North Carolina church is refusing to support a food pantry because it works with Roman Catholics because Catholics believe that
doing good things is itself a good thing.
The history of the Catholic Church in America has for a long time been one of oppression at the hands of the Protestant majority. The Ku Klux Klan opposed, in their words, "Koons, Kikes, and Katholics." The public education system was set up in a decidedly Protestant bent specifically to drive Catholics out of schools.
I'm going to use my diary to keep hammering this point home: Catholics by tradition and by current inclination should be a clear constituency of the Democratic party. In an era of close elections, this is a demographic in which we have underperformed to our detriment.