The new Republican Party Platform brought polygamy into the national political debate regarding slavery. It argued that since the Constitution gives Congress sovereign power over the territories it was "right and imperative" that Congress prohibit both slavery and polygamy in the territories. This created a dilemma the Democrats. Virtually all were offended at Mormon polygamy. The Republican Platform put the Democrats in the awkward position of explaining why a territory could become a slave state by the determination of the people, but could not self-determine the legality of polygamy.
[Hale, Van, Mormon Miscellaneous, Utah War Chronology, http://www.mormonmiscellaneous.com/utahwar/id2.html]