Missouri became the 24th state to join the Union on August 10, 1821. Missouri’s first two senators, David Barton and Thomas Hart Benton, took office on that day. Missouri’s longest-serving senators include Benton (1821-1851), who was the first senator to reach 30 years of service, and Francis Cockrell (1875-1905). Among those who rose to leadership positions is Thomas Hennings, who served as Democratic Conference secretary. As vice president of the United States, Harry S. Truman served as the Senate’s constitutional president or presiding officer.