What effect would being an ex-Mormon have on a Presidential campaign?

[Zioneer]

Well, back in 1976, there was a guy running who was close to being an ex-Mormon; Democratic Congressman Mo Udall of Arizona. He disagreed with some doctrines of the LDS Church, and from what I understand both didn't attend church anymore despite still being a member, and didn't raise his kids as Mormons.

It didn't do a thing; despite his attempts to publicly distance himself from the LDS Church, Carter's cronies (Coleman Young in particular, I think) targeted Udall on Mormonism, specifically Mormon history with blacks. I'm pretty sure Udall lost Michigan and a couple of other states on that, even though Carter had been a segregationist at one point in the past.

An interesting What-if alternate history scenario would be if the LDS Church had given the priesthood towards African-Americans earlier (part of Udall's leaving the Church was because of the black priesthood ban), and Udall goes into the 1976 primaries without that hanging around his neck.

This doesn't really answer your question, but that's the closest real-life scenario to what you're talking about, I guess.