Democratic voters aren't particularly likely to go for Catholics. For example, out of all the very many Democratic nominees for president, I believe only three have been Catholics. Currently, in the senate, 15 of the Democrats are Catholic (out of 50 senators who align with the Democrats). 75 Democrats in the House are Catholic (out of 224).
However, it's true that Democrats are more likely to vote for Catholics than Republicans are. But that's consistent with Democratic tendencies towards diversity generally. They're also more likely to vote for Jews (all the Jews in the senate align with the Democrats, and in the House there are 25 Jewish Democrats versus only 2 Republicans). They're also more likely than Republicans are to vote for Buddhists, Muslims, Hindus, Unitarians, atheists, agnostics, gays, immigrants, women, etc.) Republicans only very, very seldom will vote for anyone who isn't a heterosexual, native-born Protestant, and there aren't even many women who win office as Republicans. For example, in all of American history, every single Republican nominee for president has been a straight, white, male Protestant (assuming you consider Mormonism a form of Protestantism).
