Once again, the alleged Catholic vote

Molly Worthen's call for a stronger liberal Catholic voice in the public square is a good and thoughtful read. But it's hard to let this go by: 

Allowing Republicans to claim the mantle of Catholicism might cost the Democrats the election. As commentators have noted, Catholics may be the nation’s most numerous swing voters.

Sure, but every time a commentator notes the powerful Catholic swing vote, another commentator replies that calling a particular demographic a bloc of swing voters is a dubious move—unless they tend to all vote together, which Catholics certainly do not. Maybe their Catholicism makes them swing voters, or maybe they're simply a representative mix of Republicans, Democrats and independents who happen to be Catholic. What's the difference, for anyone except the opinion writers?