mormons in Utah were heavily Dem for quite a while.
There's actually a fascinating (but little-known) story behind early Mormon politics. In the 1840s, when the Mormons were living in Nauvoo, Illinois, Joseph Smith's brother received a revelation in which God told him to vote the Democratic ticket. Joseph publicly backed him up, saying his brother's revelations had been correct before. So, of course, virtually all Mormons voted for the Democrat.
This was the beginning of a vicious cycle. Whigs resented the bloc vote against them, and so began attacking the Mormons at election time. Of course, this only reinforced the Mormons' Democratic allegiance. Brigham Young, whose influence on the church was massive, was a lifelong Democrat; this, along with the LDS doctrine of white supremacy, led most Mormons to support the Democratic Party throughout the 19th century. Even more important was that the Republican Party was seen as less friendly to the Mormon Church, dating back to their first national platform which compared polygamy to slavery.
So, when Utah finally achieved statehood, William Jennings Bryan won a lopsided victory, with 83 percent of the vote (it didn't hurt his cause that Utah had a significant silver mining industry, either!). But with the Republican victory, LDS leaders decided that they would be better served if both parties competed for Mormon votes; so "almost at random," they told each family to vote either Democratic or Republican!
This, along with the return of relative prosperity and the end of the silver issue, meant that Utah had the biggest swing of any state in 1900, giving McKinley a majority when he hadn't cracked a fifth of the vote four years before! This was followed by the election of LDS Apostle Reed Smoot to the US Senate as a Republican, and the Democratic adoption of an anti-Mormon strategy in 1904 meant that Utah was now a Republican state; to the extent that the Beehive State voted for Taft in 1912, the only state to do so besides Vermont (the LDS President publicly said he was supporting Taft).
After that, Utah has remained generally Republican, excepting the New Deal era, becoming more-so as social issues have gained prominence.
