In 1888 Dillingham was elected the 42nd Governor of Vermont. He served the one two-year term available under the Mountain Rule, afterwards returning to the practice of law. Dillingham practiced in Montpelier with Hiram A. Huse and Fred A. Howland. Howland was Dillingham's nephew, and had studied law with Dillingham to qualify for admission to the bar. Dillingham was elected to the United States Senate in 1900, replacing temporary appointee Jonathan Ross and completing the term of Justin Smith Morrill, who had died in office. Dillingham was reelected in 1903, 1909, 1914 and 1920, and served until his death. His 1914 election was Vermont's first following passage of the Seventeenth Amendment to the United States Constitution. During his Senate career Dillingham was Chairman of the following committees: Transportation Routes to the Seaboard (Fifty-seventh Congress); Immigration (Fifty-eighth through Sixty-first Congresses); Privileges and Elections (Sixty-second, Sixty-sixth, and Sixty-seventh Congresses); and Establishing the University of the United States (Sixty-third through Sixty-fifth Congresses). Dillingham achieved prominence as the leading Progressive-era legislative spokesperson for restricting immigration. From 1907 to 1911, Dillingham chaired (concurrently with his Senate duties) the United States Immigration Commission, also called the Dillingham Commission, which concluded that immigration from southern and eastern Europe posed a serious threat to American society and culture and should be greatly reduced in the future. In 1903, Dillingham chaired a Senate subcommittee that investigated conditions in Alaska following the Klondike Gold Rush. During their trip, a new courthouse established near the Nushagak River village of Kanakanak was named in honor of the chairman and the surrounding community later adopted the name Dillingham, Alaska. While the subcommittee traveled extensively throughout Alaska, Dillingham never set foot in the Bristol Bay salmon fishing community that still bears his name. Business and civic endeavors Dillingham's business interests included serving as President of the Waterbury Savings Bank and a member of the National Life Insurance Company board of directors. He was active in the Methodist church, and served as President of the Board of Trustees of the Montpelier Seminary (originally the Vermont Methodist Seminary), and a member of the University of Vermont Board of Trustees. Dillingham was a member of the Sons of the American Revolution, and served as President of the Vermont SAR organization. Death and burial Dillingham died in Montpelier on July 12, 1923. He was buried in Waterbury's Hope Cemetery. Awards and honors Dillingham received the following honorary degrees: