Dill was elected to the U.S. Senate in 1922, beating two-term incumbent Republican Miles Poindexter. Dill campaigned as a supporter of Progressive reform and pledged to repeal the Esch–Cummins Act and push for the Columbia Basin Project. Poindexter, who was supported by major newspapers such as "The Spokesman-Review" and "The Seattle Times", attempted to portray Dill as a radical for his war record and his support of the Plumb Plan. Dill carried Spokane County, much of Eastern Washington, and the urban counties of the Puget Sound region. Dill was re-elected in 1928, but did not seek a third term in 1934. His election in 1928 marked the last time a candidate from Eastern Washington was elected U.S. Senator.