Historically, Ethnic Catholic Voters such as those Americans of Italian/Sicilian ancestry (like my family), Irish, Polish (like my wife’s family), German-Bavarians, etc were loyal Democrats. For example, JFK, an Irish-Catholic received 80% of the Catholic vote in the 1960 election. Fourty-four years later in 2004, John Kerry, also an Irish-Catholic from MA recieved 47% of the Catholic vote. So, what has happened is a gradual shift towards more conservative politics over the time period since 1960.
Why Catholic voters in Midwest states, who tend to be old line Union Catholic voters support CLinton over Obama is an interesting question for this Dem primary. However, a more interesting question is where do Catholic voters go for the General? I think in a McCain vs. Obama, McCain picks up at least what Bush did in 2004 (53%) and perhaps more.
Catholics don’t buy into guilt politics. They are born with original sin, so they have more experience with handling guilt.