The GOP was founded in 1854 by opponents of the Kansas–Nebraska Act, which allowed for the potential expansion of chattel slavery into the western territories. The party supported economic reform and classical liberalism while opposing the expansion of slavery. Abraham Lincoln was the first Republican president. Under the leadership of Lincoln and a Republican Congress, slavery was banned in the United States in 1865. The GOP was generally dominant during the Third and the Fourth Party System periods. It was strongly committed to protectionism and tariffs at its founding, but grew more supportive of free trade in the 20th century.