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, 21:35, June 4, 2007
[[Full Text of the United States Constitution|Article I, section 3]] of the Constitution requires the Senate to be divided into three classes for purposes of elections. Senators are elected to six-year terms, and every two years the members of one class-approximately one-third of the Senators-face election or reelection. Terms for Senators in Class I expire in 2013, Class II in 2009, and Class III in 2011.