The Republican Party Platform of 1856 lists polygamy as one of the "twin relics of barbarism."
"Jeffersonian Roots, Republican Party Platform, June 18, 1856," in The Republican Party: Documents Decoded, ed. Douglas B. Harris and Lonce H. Bailey (Santa Barbara, CA: ABC-CLIO, 2014), 4
Resolved: That the Constitution confers upon Congress sovereign powers over the Territories of the United States for their government; and that in the exercise of this power, it is both the right and the imperative duty of Congress to prohibit in the Territories those twin relics of barbarism — Polygamy, and Slavery.