“Many” is an ambiguous term. Mormons vote 4 to 1 Republican. And yes, there may be some Republican proponents of polygamy, but not a whole lot.
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Mormons CLAIM to vote 4:1 Rep. They are more likely to vote for a liberal member of the Church than a non-member conservative.
And many of the LDS i know that believe the doctrines above were registered republican but still supported Abortion, Gay Marriage and social programs. Remember Mitt is registered Republican as well.
When I lived in Utah (Provo/Orem, I would say about 60-70% of the LDS I knew (in the several hundreds) supported at least one of the liberal views I mentioned. The support for abortion was higher in my ward in CA than my ward in Utah, and the support for polygamy and socialism was higher in Utah than CA.
I'd put Mitt in a similar category with Reid. In fact, I'd put most politicians in that category.
If Mormons wanted to vote for the socialist/gay marriage/abortion candidate, then all they would have to do is vote for the Democrat. But they don't.