Yeah in that time frame I don't see a president converting benefiting anyone without having a tl where the Mormons don't get chased out of every place they stop between new York and Utah (exageration, but work with me). Even then, the mormon theology is so different from protestant or catholic theologies that I don't see it workin well in those days.
Having an ex-president convert could have interesting implications in the long run though. Let's say a president converts some time after leaving office, and makes a few public appeals to the religion. They focus on thing like the constitution being godinspired or holy (I think it's one of those) and play down polygamy as a minority thing frowned upon by the mainstream(I don't know when the Mormon church aband one polygamy) he could make it more palatable to many people. Being assassinated could actually be a huge assett to the Mormons for proselytizing in the US, especially if he is killed by a KKK member or some other group that is easy to portray as a villain of the state/public. If they were popular while in office, and charasmatic enough, I could see mormonism being commonly accepted much earlier, and being a visible minority in many more places than it is. Uhhhh, that being said, Mitt Romney wasn't really hampered by his faith when he ran.....right? If so, how early could a mormon seriously run and win? When otl did it become mostly accepted?