Hoopes became a member of the Socialist Party of America in 1914. During the 1920s he served as secretary of the Socialist Party of Pennsylvania. Hoopes served as a member of the national Socialist executive committee during the 1930s. Hoopes was selected to serve as the chairman of the Socialist Party of America's 1934 national convention by a vote of 69 to 61 against George E. Roewer. He served as chairman of the Socialist Party of America from 1946 to 1968. Hoopes joined the Socialist Party USA in 1973.