The Dawes Act was written by a Republican senator, Henry L. Dawes. Because the Republicans had the majority in the Senate at the time, he was able to get his legislation approved in 1887. The basic intention of the Republican Party in passing this bill was to ensure that Native Americans ("Indians") had permanent legal ownership of their land. The Dawes Act was amended by subsequent pieces of legislation over the next two decades.
The 1887 legislation had mixed results. One the one hand, it clearly established legal ownership for Indians. On the other hand, it created a tangled of regulations regarding who had legal jurisdiction over that land, and weakened the role of tribal governments.
Further progress was made with the Indian Citizenship Act of 1924. With both the House of Representatives and the Senate featuring Republican majorities, and with Republican President Calvin Coolidge in the White House, it was finally possible to grant full citizenship to Native Americans.