Allison practiced law in his hometown of Ashland, Ohio, and (from 1857) in Dubuque, Iowa. In the House of Representatives he was a partisan of railroad interests, but as a senator he helped to pass the Hepburn Act (1906), a railway rate-regulation bill, on behalf of President Theodore Roosevelt. Although identified with the silver-purchase act, which he cosponsored, he was more important as a member of the powerful House Ways and Means Committee and as chairman of the Senate Committee on Appropriations. With Nelson W. Aldrich, Orville H. Platt, and John C. Spooner, he made up “the Four” who dominated the Senate in the early 1900s. He twice (1888, 1896) was a contender for the Republican presidential nomination.