Can you cite where there is a religious litmus test when selecting a POTUS?
That isn't the point.
The thing is, there are some religions that are just so far out there that people won't vote for them.
We'd never elect a Muslim President. Or a Buhddist. Or a Hindu. As much as it distresses my non-believing self, we'd never elect an Atheist. All 44 presidents have been some flavor of Christian, and that's not going to change. Maybe, maybe a Jew could get elected. They are just within the comfort zone.
And on top of that, they have to be a mainstream Christian, not a member of a whacky offshoot with whacky beliefs. We'd never elect a Jehovah's Witness, an Amish, or some other sect outside the mainstream. And frankly, they are a lot more mainstream than the Mormons are. They don't make up their own bibles to justify their beliefs, they just interpret the one everyone else has a bit differently.
The Mormons have spent a lot of time trying to wallpaper over the whackiness. They don't tell you about the Spirit Children and the Nephites and the Magic Underwear up front. But it's still there, and it will get a lot of attention if the Republicans are dumb enough to nominate Romney.