An enthusiastic Republican supporter before the Civil War, Allison served as Iowa as a Congressman (1863-71) before moving on the U.S. Senate where he served from 1873 until his death. During his time in the Senate, he became identified with the nationwide expansion of business, particularly in the manufacturing and transportation sectors. He was considered one of the Senate's experts in finance, with the result that, for over 25 years, he served as the Appropriation Committee's chairman and had a seat on the Finance Committee. At his death, he was the senior member of the U.S. Senate, and a Republican institution, Allison's being largely responsible for the direction the party had taken up to that time.