Classes of United States Senators

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 so that one of them is up for election every two years, rather than having all 100 seats up for election at once. For example, the 33 Senate seats of Class 1 were elected in 2012, the 33 seats of Class 2 were up for election in 2014, and the 34 seats of Class 3 were up for election in 2016.

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 senator is the one with the greater seniority in the Senate. This is mostly based on length of service.

When the Founding Fathers agreed to give six-year terms to Senators, they also decided to stagger the elections, so that a third of the Senate was up for election every two years. With this staggered turnover, the Founding Fathers wanted to ensure stability in the Senate, and encourage Senators to deliberate measures over time, rather than risk a rapid turnover of the entire chamber every six years. At the same time, they wanted more frequent elections, as opposed to waiting every six years, to prevent Senators from permanently combining for "sinister purposes".

The three classes of the Senate were then specified by Article I, Section 3 of the U. S. Constitution:

Immediately after they shall be assembled in Consequence of the first Election, they shall be divided as equally as may be into three Classes. The Seats of the Senators of the first Class shall be vacated at the Expiration of the second Year, of the second Class at the Expiration of the fourth Year, and of the third Class at the Expiration of the sixth Year, so that one third may be chosen every second Year.

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