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 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.
Historical division
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 promote 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 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:
This was achieved 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, because of its late ratification, had not yet selected its senators. 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. In accordance with their recommendation, on May 14 the Senate divided the members into three classes:
On the following day, May 15, the term expiration of each class was determined by drawing lots. Lot 1 was drawn by Dalton, 2 by Wingate, and 3 Langdon.
Upon the expiration of a senator's term of any length, someone starts a new six-year term as senator.
Because each state is represented by two senators, regardless of population, each Senate class collectively represents a different number of people than each other Senate class. Since the early nineteenth century, class 2 senators have cumulatively represented between 50–60% of the population of the United States, while senators from each of the other two classes have cumulatively represented approximately 70–75% of the population of the United States. Several currently large states, including California, New York, Florida, Pennsylvania, and Ohio, all have their senators in classes 1 and 3, helping to create this imbalance.
New states
When a new state is admitted to the Union, its two senators have terms that correspond to those of two different classes, among the three classes defined below. Which two classes is determined by a scheme that keeps the three classes as close to the same size as possible; one that avoids the largest class differing by more than one senator from the smallest class. A coin toss determines which new senator enters which of the classes selected to be expanded.This means at least one of any new state's first pair of senators has a term of more than two and up to six years, and the other has a term that is either two or four years shorter.
When the last state, Hawaii, was admitted in 1959, candidates for the Senate ran either for "seat A" or "seat B". The new senators, in a process managed by the Secretary of the Senate, drew lots to determine which of the two would join the class 1, and which would join class 3.
Should a 51st state be admitted, it would receive senators in classes 1 and 2, at which point all three classes would have 34 senators.
Class 1
Class 1 consists of:- the 33 current senators whose seats are scheduled for re-election in November 2024, and whose terms end January 3, 2025; and
- earlier senators with terms that ended in 1791, 1797, 1803, 1809, 1815, 1821, 1827, 1833, 1839, 1845, 1851, 1857, 1863, 1869, 1875, 1881, 1887, 1893, 1899, 1905, 1911, 1917, 1923, 1929, 1935, 1941, 1947, 1953, 1959, 1965, 1971, 1977, 1983, 1989, 1995, 2001, 2007, 2013, and 2019.
Class 2
Class 2 consists of:- the 33 current senators whose seats are scheduled for re-election in November 2020, and whose terms end January 3, 2021; and
- earlier senators with terms that ended in 1793, 1799, 1805, 1811, 1817, 1823, 1829, 1835, 1841, 1847, 1853, 1859, 1865, 1871, 1877, 1883, 1889, 1895, 1901, 1907, 1913, 1919, 1925, 1931, 1937, 1943, 1949, 1955, 1961, 1967, 1973, 1979, 1985, 1991, 1997, 2003, 2009, and 2015.
Class 3
Class 3 consists of:- the 34 current senators whose seats are scheduled for re-election in November 2022, and whose terms end January 3, 2023; and
- earlier senators with terms that ended in 1795, 1801, 1807, 1813, 1819, 1825, 1831, 1837, 1843, 1849, 1855, 1861, 1867, 1873, 1879, 1885, 1891, 1897, 1903, 1909, 1915, 1921, 1927, 1933, 1939, 1945, 1951, 1957, 1963, 1969, 1975, 1981, 1987, 1993, 1999, 2005, 2011 and 2017.
Election cycle years
| Class | Most Recent Election Year | Next Scheduled Election Year |
| Class 1 | 2018 | 2024 |
| Class 2 | 2014 | 2020 |
| Class 3 | 2016 | 2022 |
List of current senators by class
The following table lists the senators by party by class.The following table lists the senators by state and by class, including the states' Cook Partisan Voting Index ratings, which indicate the party direction in which a state tends to lean and the extent of that lean. |