Classes of United States Senators
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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.
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.
This was achieved in May 1789, several weeks after the first Senate assembled. The Senate first divided the members into three classes, on May 14.
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, Mr. Johnson, Mr. Morris, Mr. Henry, Mr. Izard, and Mr. Gunn;
- The second of Mr. Wingate, Mr. Strong, Mr. Paterson, Mr. Bassett, Mr. Lee, Mr. Butler and Mr. Few;
- And the third of Mr. Dalton, Mr. Ellsworth, Mr. Elmer, Mr. Maclay, Mr. Read, Mr. Carroll, and Mr. Grayson.
That three papers of an 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, in 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 have not 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.
The Senate proceeded to determine the classes, agreeably to the resolve of yesterday, on the mode of carrying into effect the provision of the second clause of the third section of the first article of the constitution, and the numbers being drawn, the classes were determined as follows:
Lot No. 1, drawn by Mr. Dalton, contained Mr. Dalton, Mr. Ellsworth, Mr. Elmer, Mr. Maclay, Mr. Read, Mr. Carroll, and Mr. Grayson, whose seats shall, accordingly, be vacated in the Senate, at the expiration of the second year.
Lot No. 2, drawn by Mr. Wingate, contained Mr. Wingate, Mr. Strong, Mr. Paterson, Mr. Bassett, Mr. Lee, Mr. Butler, and Mr. Few, whose seats shall, accordingly be vacated in the Senate, at the expiration of the fourth year.
Lot No. 3, drawn by Mr. Langdon, contained Mr. Langdon, Mr. Johnson, Mr. Morris, Mr. Henry, Mr. Izard, and Mr. Gunn, whose seats shall, accordingly, be vacated in the Senate, at the expiration of the sixth year.
Upon the expiration of a senator's term of any length, someone starts a new six-year term as senator (based on election by the state legislatures until the Seventeenth Amendment required direct popular election of Senators).
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 less than six years, and one term is either two or four years shorter than the other.
When the last state was admitted, Hawaii 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 (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).
Class 1 
Class 1 consists of:
- the 33 senators who were elected in November 2012, whose seats are scheduled for re-election in November 2018, and whose terms began in January 2013 and will end in January 2019; and
- earlier senators with terms ending 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, and 2013.
States with a Class 1 senator: Arizona, California, Connecticut, Delaware, Florida, Hawaii, Indiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Jersey, New Mexico, New York, North Dakota, Ohio, Pennsylvania, Rhode Island, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin, and Wyoming.
Class 2 
Class 2 consists of:
- the 33 current senators whose seats are scheduled for re-election in November 2014, and whose terms end in January 2015; 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, and 2009.
States with a Class 2 senator: Alabama, Alaska, Arkansas, Colorado, Delaware, Georgia, Idaho, Illinois, Iowa, Kansas, Kentucky, Louisiana, Maine, Massachusetts, Michigan, Minnesota, Mississippi, Montana, Nebraska, New Hampshire, New Jersey, New Mexico, North Carolina, Oklahoma, Oregon, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Virginia, West Virginia, and Wyoming.
Class 3 
Class 3 consists of:
- the 34 current senators whose seats are scheduled for re-election in November 2016, and whose terms end in January 2017; 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, and 2011.
States with a Class 3 senator: Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Florida, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Missouri, Nevada, New Hampshire, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, South Dakota, Utah, Vermont, Washington, and Wisconsin.
List of current Senators by Class
- "The Senate and the United States Constitution". senate.gov. Retrieved 2011-10-23.
- "Journal of the Senate of the United States of America". Journal of the Senate of the United States of America, 1789-1793. Library of Congress. May 14, 1789. Retrieved June 11, 2013.
- "Journal of the Senate of the United States of America". Journal of the Senate of the United States of America, 1789-1793. Library of Congress. May 15, 1789. Retrieved June 11, 2013.
- "Frequently Asked Questions about a New Congress". United States Senate. Retrieved 11 June 2013.
- Davies, Lawrence E. (July 30, 1959). "G.O.P. Wins Governorship in Hawaii's First State Vote". The New York Times.
- Trussell, C. P. (August 25, 1959). "Congress Hails Three New Members from 50th State". The New York Times.
- US Senate class page (old)
- Current Class 1, (senate.gov)
- Current Class 2, (senate.gov)
- Current Class 3, (senate.gov)