It shouldn't. It really seriously shouldn't, since we at least pretend that Church and State are kept separate around here.
BUT....that's the thing isn't it? Religion doesn't seem to come up unless people sincerely feel threatened by it. And yes, that happened once on the lefty side with JFK being Catholic. But how many times does it happen with more fundamentalist sorts on the right-ward side? Really, it seems like every time someone of a conservative sort _runs_ (since the 1980s at least), they _have to_ suck up to your Billy Graham crowd. And then people act like that's "normal", but then freak out over Kennedy's church or Obama's church.
See what I'm saying? There's more than a bit of a double standard going on with this--so much so that lots of people are led to assume that conservative instantly equals fundie Baptist or some sort. As an answer stated above me, declaring atheism becomes political suicide--but only from the right. On the left going to a church _that dares challenge the right_ is what becomes political suicide.
Really. That's the honest truth here. It shouldn't matter but it does. I blame Senator McCarthy--you know, the guy who put "Under God" in the Pledge of Allegiance in the 1950s? Him.