Man, oh man, I hate this ad:
From a pure politics perspective, fair play is fair play: Republicans have spent a couple of generations using religion as a cudgel to portray Democrats and liberals as un-Christian and un-American, and it's a strategy they continue to pursue. Goose, gander, etc.
But liberals have mostly resented this line of attack, believing—rightly, I think—that it veers pretty closely to violating the spirit of the Constitution's "no religious test" for candidates for public office. Creating and running this ad means we've decided the principle isn't so important after all—if we can find a way to use religion as a cudgel from a position of strength, we'll do it!
And I can't help but think that weakens the foundation of our ability to defend the rights of religious minorities—Muslims, Jews, atheists, agnostics, and so forth. Maybe it's fair play; I'm not at all sure it's wise.
UPDATE: And politically, it's not that smart either. If Democrats get into a pissing match with Republicans about who is most Christian, Democrats are mostly going to lose.