Senators, 1789 to present
From 1789 to 1958, senators whose terms began on the same day are listed alphabetically. Beginning in 1959, senators are listed according to commencement of first Senate term by order of service, determined by former service in order as senator, vice president, House member, cabinet secretary, governor, and then by state population. This latter system for calculating order of service has been used by the modern Senate for many years for the purposes of office assignment. It is unclear just when the Senate first began applying such criteria.
A Chronological Listing of U.S. Senators