I suspect that Romney's religious affiliation isn't going to change many people minds to vote for the other team.
I seriously doubt that many evangelical christians (who think that Mormonism isn't a Christian faith) are going to cast their votes for Obama.
Of course they might just NOT vote at all, so that might effect the outcome.
Or they might vote third party.
But I think the bigger danger (to GOP chances) is the person who right now doesn't really know all that much about Mormonism and just how strange their beliefs actually are. The media will happily educate them.