I think that Feingold's chances in 2008 have less to do with his faith than with his political views and his abilities as a campaigner. Adding Joe Lieberman to the ticket initially helped Gore in 2000, in part because Lieberman was seen as devout and sincere in his religious beliefs.
Now an atheist president -- that's something you're not going to see in this country for a long time.
The American Conservative Union gave him a rating of 72 for 2004, 75 for 2003. That's a long way from being a Democrat but puts him to the left of all other AZ Congressional Republicans except Jim Kolbe.