I have heard that Mormons are around 30%. That’s enough to make the difference, since Catholics, Protestants, and none-of-the-above will presumably vote along lines of maybe 55-45% at best.
It’s like the black vote. Romney would have at least 90% of the Mormon vote in his pocket, and they would all come out if ordered to.
There are around 350,000. That's very far from 30%.