The only group that's more partisan is members of historically black churches, according to Pew, 77 percent of whom identify themselves as Democrats. (Though that's more of a racial subset of a religious category, than it is a religious category in its own right.)
Perhaps unsurprisingly, Mormons are much more socially conservative than both the general population and other groups: 70 percent say abortion should be illegal in all or most cases, compared to 42 percent of the general population and 35 percent for Evangelicals. 68 percent say homosexuality should be discouraged, rather than accepted--that's in line with evangelicals (64 percent) and Muslims (61 percent)--hence the LDS church's involvement in Prop. 8--but lower than the general population, 40 percent of which shares that opinion.
Mormons make up 1.7 percent of the nation; 35 percent of them live in Utah. In that regard, one could say they're overrepresented in the national political elite, as both Mitt Romney and Senate Majority Leader Harry Reid are Mormons. Numbers-wise, Mormons aren't a huge voting bloc--but, as we saw with Prop 8, they can be politically powerful: supporters of the gay marriage ban estimated in October that Mormons had donated 30 to 40 percent of the $25.5 million raised to back it; Advocate.com estimated that figure at 77 percent.