classes

The 100 U.S. Senate seats are classified into three classes of United States senators, two of which (classes 1 and 2) consist of 33 seats and one (class 3) of 34 seats. The classes determine which Senate seats will be up for election in any two-year cycle, with only one class being up for election at a time. Elections for class 1 seats took place in 2018, class 2 in 2020, and the elections for class 3 seats 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 proviso 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 number as possible.A senator's description as junior or senior senator is not related to their class. Rather, a state's senior U.S. senator is the one with the greater seniority in the Senate, which is mostly based on length of service.

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