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 so that one of them is up for election every two years, rather than having all 100 seats up for election at once. For example, the 33 Senate seats of Class 1 were elected in 2012, the 33 seats of Class 2 were up for election in 2014, and the 34 seats of Class 3 are scheduled for re-election in 2016.