Order of service is based on the commencement of the senator's first term. Behind this is former service as a senator (only giving the senator seniority within his or her new incoming class), service as vice president, a House member, a cabinet secretary, or a governor of a state. The final factor is the population of the senator's state.A Chronological List of United States Senators 1789-Present, via www.Senate.gov1871 U.S Census Report Contains 1870 Census results1881 U.S Census Report Contains 1880 Census results1891 U.S Census Report Contains 1890 Census results
