This Pew Forum squib provides some nice graphics about trends in religious groups’ candidate preferences. The McCain/Obama 2008 split is looking very much like the Bush/Kerry 2004 split. Indeed, as many have remarked, the 2008 electoral map looks quite similar to the 2004 map, with the same swing states and the same swing constituencies.
I’ve said it before and I’ll say it again: if you view the electorate through a religious prism: American Catholics are the crucial swing constituency. Obama had trouble with them in the primary season, and he’s having trouble with them now. Joe Biden, Catholic though he is, doesn’t help, for reasons that are obvious to any observant and obedient Catholic.
The McCain campaign has to hope that the Catholic swing toward him (or away from Obama) continues; it will make those battleground states a little more hospitable.