I also recognize that the Roman Catholic Church on economic issues has what might be charitably called a medieval and paternalistic view of the role of the state. There are important areas of disagreement even there, and they came out when Rick Santorum’s views on economics got criticized for being out of line with standard conservative evangelical views and even more different from secular conservative or libertarian views.
However, would you not make a distinction between the general population of people who call themselves Catholics and those who are committed Mass-going Catholics? We can't compare all Catholics to churchgoing evangelical Protestants without recognizing that there are major differences in commitment levels within Roman Catholicism, just as there are major differences within Protestantism.
A practicing Catholic who goes to Mass regularly and cares about what his church teaches is going to be much more similar to an evangelical Protestant in voting patterns.
Granted, evangelicals have their Ron Siders and Tony Campolos, and seriously committed Catholics have their social liberals, too.
But I don't see social liberals as being anywhere close to a majority of practicing Roman Catholics who take their faith seriously.
I’m not interested in internal religious purity of individuals within denominations, I am interested in a denomination voting democrat, in an almost perfect record for 160 years.
A powerful, large denomination, and one which has fought to pass laws to import 10s of millions of more democrat voters, to vote democrat.
The non-Catholic, Christian vote, as loose, diverse, and varied a category as one can make, has only gone democrat 3 times in history, 1932, 1936, and 1964.