what fraction of the senate is up for reelection at one time
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 will be up for election in 2018, the elections for the 33 seats of Class 2 will take place in 2020, and the elections for 34 seats of Class 3 will be held in 2022.