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The three classes of US Senators, each currently including 33 or 34 Senators (since Hawaii became the 50th state in 1959, and until another state is admitted), are a means used by the United States Senate for describing the schedules of Senate seats' elections, and of the expiration of the terms of office of the Senators holding the respective seats. Each numbered Congress sits for two years, and each Senator normally serves a term of six years in office.

Senatorial classes do not identify who each state's Senior and Junior Senator is - that is determined by comparing the tenures of each state's current Senators, regardless of class.

Historical division

Respecting the members of the Senate, Article I, Section 3 of the U. S. Constitution specifies that:

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 actually achieved several weeks after the first Senate assembled. From the Journal of the Senate of the United States of America, 1789-1793:

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 scats 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.

And from the Journal of Friday, May 15, 1789:

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. Landon, 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, i.e., that avoids any class differing by more than one from the minimum-sized class.

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.

Should a 51st state be admitted to the Union, it would receive Senators in Classes I and II, and at that point all three Classes would have 34 Senators each time around.

Classes

     Classes 1 and 2      Classes 1 and 3      Classes 2 and 3

Class I

Class I consists of

• the 33 current Senators whose seats are scheduled for re-election in November 2012, and whose terms end in January 2013; and
• earlier Senators with terms ending in 2007, 2001, 1995, 1989, 1983, 1977, 1971, 1965, 1959, and back to 1791; and
• Current States with a Class I 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 II

Class II consists of

• the 33 current Senators whose seats were scheduled for re-election in November 2008, and whose terms end in January 2009; and
• earlier Senators with terms that ended in 2003, 1997, 1991, 1985, 1979, 1973, 1967, 1961, and back to 1793; and
• Current States with a Class II 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 III

Class III consists of

• the 34 current Senators whose seats are scheduled for re-election in November 2010, and whose terms end in January 2011; and
• earlier Senators with terms that ended in 2005, 1999, 1993, 1987, 1981, 1975, 1969, 1963, and back to 1795; and
• Current States with a Class III 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.

Current Senators in each class by party

In the 110th United States Congress:

External links

• US Senate class page (old)
• Current Class I, (senate.gov)
• Current Class II, (senate.gov)
• Current Class III, (senate.gov)