You are correct. Just under 50% of Catholics voted for Bush in 2000. Cultural Catholics in unions?
The best that I can tell, Catholics have only voted Republican six times in American history, five of those times have been for an already incumbent Republican running for reelection, and one of those six times is disputed by Gallup as going Republican, (1956).
None of those votes were by huge margins, 54% was the largest. Four of them were under Pope John II, and three of them were for Reagan or his veep during that period.
Five of the six times that Catholics voted Republican took place after boomers became old enough to vote.
The total Protestant vote has only gone Democrat in 1932, 1936, and 1964. The Protestant Hispanic vote is to the right of the total Catholic vote, going Bush by 56% in 2004 and for Obama by only 52% in 2008, (Total Catholic vote was Bush 52%-2004, and Obama 54%-2008) so I think that this is about more than a few union people.