Immediately after they shall be assembled in Consequence of the first Election, they shall be divided as equally as may be into three Classes. [U.S. Constitution, Article I, section 3, clause 2]
Every two years, one-third of the Senate’s members are elected (or reelected). The Constitution’s framers based this three-class system on precedents established by state governments. Delaware’s senate and Pennsylvania’s unicameral council were divided into three classes on a one-year election rotation, while upper houses in Virginia and New York had four classes and yearly elections.
At the 1787 Constitutional Convention, delegates debated both the Senate class system and the issue of length of terms. On June 25, Massachusetts delegate Nathaniel Gorham suggested a four-year Senate term with one-fourth of the senators elected each year. Edmund Randolph of Virginia supported a staggered rotation in the Senate with seven-year terms. Hugh Williamson of North Carolina countered that six-year terms were more easily divisible into equal election cycles than seven-year terms. The following day, Gorham called for a six-year term, “one third of the members to go out every second year.” After further debate, delegates adopted the six-year, three-class Senate by a vote of 7 to 4.
Framers hoped that staggered elections would bring stability to the Senate, and in turn, to other branches of the new government. Class rotations would bring about gradual change, they argued, and prevent senators from permanently combining for “sinister purposes” while encouraging senators to deliberate measures over time.
At the start of the first session of Congress in 1789, senators were divided into the three classes by lot with same-state senators assigned to separate groups. The term for the first class expired in two years, the second in four years, and the third in six years. Subsequent elections to all classes were for a full six-year Senate term.
The rotation of senators, and the fact that two thirds of its members carry over from Congress to Congress, established the idea that the Senate is a “continuing body”—unlike the House of Representatives, whose entire membership faces election every two years. Unlike the House, the Senate does not adopt a new set of rules at the start of each Congress.