In 1926, Steiwer was elected as a Republican to the U.S. Senate, defeating incumbent Robert Stanfield in the primary. He won with only 39% of the vote, running against Democrat and later judge Bert Haney and Stanfield, then running for re-election as an independent. In 1928, he was one of many candidates for the Republican Presidential nomination at the National Convention, with Herbert Hoover winning the nomination and then the fall election. At the 1936 Republican National Convention he was the keynote speaker and temporary chairman, as well as an unsuccessful candidate for the nomination. Steiwer was re-elected in 1932 and served from March 4, 1927 until January 31, 1938, when he resigned to return to the practice of law after suffering health problems. He had undergone gall bladder surgery in November 1936.