I think a president converting in that era would cause, if not a successful impeachment, but a complete agreement with states who withdrew from the union and the congress would let them go.
FYI: Polygamy was given up for Utah statehood.
Abraham Lincoln freed only the slaves in the states that had withdrawn from the union. He was hoping for an uprising to support his cause, not doing it out of the goodness of his heart or because he thought owning human beings was wrong.
If any President converted to something 'unacceptable' while in office (Mormonism, Buddhism, Hinduism, or some of the odder Christian offshoots, I think there would be a huge furor, and that he'd seriously damage his party's standing. Thus, I suspect what he'd do is not formally convert until after his term was over. Note that even Tony Blair thought it more politic to wait until after stepping down before converting (officially) to Roman Catholicism, and that was in the 21st century!!
That's totally ASB and more than a little insulting. Mormonism is not simply some sort of "free-love" communal religious movement, and no one would convert to it so they could have more wives.how about if the president clearly states that the only reason that he converted was because they support polygamy? more political uproar?
I do not see how Lincoln's OTL motivation for the Emancipation Proclamation means that an ATL Apostle Lincoln would not be anti-slavery.
What about widowed president convert because he fall in love with Mormon women ? ' i love both Smith sisters and i want to marry them both'. A man condemned in 19th century and rehabilitated in 21st century ? The most romantic President, who impeached for love.