What did the strong support of the Republican Party in the Election of 1856 prove? A) The anti-slavery movement in the United States was on the decline. B) The anti-slavery movement in the United States was growing. C) Slavery was no longer an important issue to the citizens of the United States. D) The building of a railroad was the most important issue facing the United States. What did the strong support of the Republican Party in the Election of 1856 prove?