As a Catholic, you know that Mormonism is a non-Christian cult.
Mormonism is an anti-Christian religion, founded on converting Christians, mostly with clandestine means, what they call milk before meat, where they convince the (often Catholic) Christian that he or she is merely moving sideways, moving into another Christian denomination.
Actually that person is moving into Bishop Romney’s, antiChristian religion, a move which will require baptism into Mormonism.
As far as Catholics are concerned Mormonism is a Christian heresy, as heretical as any other non-Catholic church (save perhaps the Orthodox) as far as I know, as the church has never provided a measure of degrees of heresy.
And the church has never had any official standard of the Christianity of other churches either. If the Mormons want to call themselves Christian that is entirely their business. If they say they are Christian, they are heretical, if they are not Christian then they are not Christian.
If they are not Christian (a broad definition), that is just as much or as little a problem as being a Hindu or Buddhist, or Lutheran or Baptist for that matter. Catholics have no requirement binding them to vote for other Catholics just because they are Catholics. The church would not oppose the election of a Buddhist, why should it oppose a Mormon ?
As far as conversions - the current policy of the church, again as I understand it, is that in the political realm it does not require that other faiths be prevented from converting Catholics as long as Catholics are free to convert others as well. The major loss to conversion is actually from Evangelical/Pentecostal groups, not the Mormons.