Constitutional footing

The U. S. Constitution sets the fixed term of senators to 6 years and staggers their elections into three cycles, so that 1/3 of the Senate was up for election every 2 years. This allows at least some Senate elections to be held during any presidential or midterm election year, as the U.S. President is elected to a fixed term of four years and members of the U.S. House of Representatives are elected to fixed terms of two years. The objective is to promote stability in the Senate, and encourage senators to deliberate measures over time, rather than risk a rapid turnover of the entire chamber every 6 years. At the same time, it provided for more frequent elections as opposed to waiting every 6 years, to prevent senators from permanently combining for "sinister purposes".^[1]

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

Immediately after they'll be assembled in Consequence of the first Election, they'll be divided as equally as may be into 3 classes. The Seats of the Senators of the first Class shall be vacated at the Expiration of the 2nd Year, of the 2nd Class at the Expiration of the 4th Year, and of the 3rd Class at the Expiration of the 6th Year, so that one-third may be chosen every 2nd Year.

The allocation took place in May 1789, several weeks after the first Senate assembled. Only 20 senators from 10 states were present; North Carolina and Rhode Island had not yet ratified the U.S. Constitution, and New York hadn't yet selected its senators because of its late ratification.^[4] To decide on how to implement the division into classes, on May 11, the Senate appointed a committee consisting of Senators Ellsworth, Carroll, and Few.^[5] In accordance with their recommendation, on May 14 the Senate divided the members into three classes:^[6]

Thursday, May 14, 1789. The committee appointed to consider and report a mode of carrying into effect the provision in the second clause of the third section of the first article of the Constitution, reported:

Whereupon, Resolved, That the Senators be divided into three classes:

• The first to consist of Mr. Langdon [of New Hamphire], Mr. Johnson [Connecticut], Mr. Morris [Pennsylvania], Mr. Henry [Maryland], Mr. Izard [South Carolina], and Mr. Gunn [Georgia];
• The second of Mr. Wingate [of New Hamphire], Mr. Strong [Massachusetts], Mr. Paterson [New Jersey], Mr. Bassett [Delaware], Mr. Lee [Virginia], Mr. Butler [South Carolina], and Mr. Few [Georgia];
• And the third of Mr. Dalton [of Massachusetts], Mr. Ellsworth [Connecticut], Mr. Elmer [New Jersey], Mr. Maclay [Pennsylvania], Mr. Read [Delaware], Mr. Carroll [Maryland], and Mr. Grayson [Virginia].

That 3 papers of equal size, numbered 1, 2, and 3, be, by the Secretary, rolled up and put into a box, and drawn by Mr. Langdon, Mr. Wingate, and Mr. Dalton, on behalf of the respective classes in which each of them are placed; and that the classes shall vacate their seats in the Senate according to the order of numbers drawn for them, beginning with number one: And that, when Senators shall take their seats from States that haven't yet appointed Senators, they shall be placed by lot in the foregoing classes, but in such manner as shall keep the classes as nearly equal as may be in numbers.

On the next day, May 15, the term expiration of each class was determined by drawing lots.^[6] Lot 1 was drawn by Dalton, 2 by Wingate, and 3 by Langdon.

Upon the expiration of a senator's term of any length, someone starts a new 6-year term as senator (based on election by the state legislatures until the Seventeenth Amendment required direct popular election of senators).

Addition of new states to the Union

When a new state is admitted to the Union, its two senators are placed into separate classes. Which two classes are determined by a scheme that keeps the three classes as close to the same size as possible, so that the largest class never differs by more than one senator from the smallest class.^[2] A random draw determined which new senator enters which of the classes selected to be expanded.^[2] This means at least one of any new state's first pair of senators had a term of more than 2 and up to 6 years and the other had a term that was 2 or 4 years shorter.

New York, which held its first Senate elections in July 1789, was the first state to undergo this process after the original May 1789 draw by the Senate of the 1st Congress. Among the new senators, Philip Schuyler drew the lot for class 1 (whose term would end in 1791) while Rufus King drew class 3 (whose term would end in 1795).^[7] This made class 1 have 8 senators while classes 2 and 3 had 7 senators each. North Carolina was then assigned classes 2 and 3 after holding its first Senate elections in November 1789, making all three classes have 8 seats each.

When the newest state, Hawaii, was admitted in 1959, its first Senate elections had candidates run either for "seat A" or "B". The new senators, Hiram Fong and Oren E. Long, in a process managed by the Secretary of the Senate, drew lots to determine which of the two would join class 1 (whose term would end in five-and-a-half years), and which would join class 3 (whose term would end in three-and-a-half years).^[4][8][9] If a 51st state is admitted, it will receive senators in classes 1 and 2, at which point all three classes would have 34 senators.^[2]

Because each state is represented by 2 senators, regardless of population, each class varies in electorate and populace. Since the early 19th century, it so happens class 2 senators cumulatively co-represent 50–60% of the population; senators from each of the other 2 classes: 70–75% of the U.S. population. ^[10] Because each state has 2 senators, the sum of these figures is 200%, not 100%. Several of the most populous states, such as California, Florida, New York, Pennsylvania, and Ohio, have their senators in classes 1 and 3, provoking this imbalance.

The only times when both of a state's Senate seats are up for election in the same year are either when a new state joins the union (as mentioned above), or when there is a special election to fill a vacant seat. Special elections have no bearing on when the term for that seat ends, and a senator elected in a special election will serve the remainder of the term, until the next regularly scheduled election, not affecting which class that seat falls within.^[3]

Comparison with other United States general elections