Classes of United States senators
Class 1 Class 2 Class 3 Total
Democratic 21 12 12 45
Republican 10 21 22 53
Independent 2 0 0 2
Last election 2018 2014 2016
Next election 2024 2020 2022
TOTAL 33 33 34 100
The three classes of United States senators are made up of 33 or 34 Senate seats each. The purpose of the classes is to determine which Senate seats will be up for election in a given year. The three groups are staggered such that senators in only one of the classes are up for election in any two-year cycle, rather than having all 100 seats up for election at once. Thus, the 33 Senate seats of class 1 were up for election in 2018, the elections for the 33 seats of class 2 will take place in 2020, and the elections for the 34 seats of class 3 will be held in 2022. The three classes were established by Article I, Section 3, Clause 2 of the U.S. Constitution. The actual division was originally performed by the Senate in May 1789 by lot, with a rule being that a state's two seats had to be in different classes. Whenever a new state subsequently joined the union, its two senate seats were permanently assigned to two different classes by coin toss, while keeping the three classes as close to the same size as possible. A senator's description as junior or senior senator is not related to his or her class. Rather, a state's senior U.S. senator is the one with the greater seniority in the Senate. This is mostly based on length of service.