Classes of United States senators
State Class 1 Class 2 Class 3 Cook PVI
Alabama - Doug Jones ( D ) Richard Shelby ( R ) R+13
Alaska - Dan Sullivan ( R ) Lisa Murkowski ( R ) R+9
Arizona Kyrsten Sinema ( D ) - Martha McSally ( R ) R+5
Arkansas - Tom Cotton ( R ) John Boozman ( R ) R+15
California Dianne Feinstein ( D ) - Kamala Harris ( D ) D+12
Colorado - Cory Gardner ( R ) Michael Bennet ( D ) D+1
Connecticut Chris Murphy ( D ) - Richard Blumenthal ( D ) D+6
Delaware Tom Carper ( D ) Chris Coons ( D ) - D+6
Florida Rick Scott ( R ) - Marco Rubio ( R ) R+2
Georgia - David Perdue ( R ) Kelly Loeffler ( R ) R+5
Hawaii Mazie Hirono ( D ) - Brian Schatz ( D ) D+18
Idaho - Jim Risch ( R ) Mike Crapo ( R ) R+19
Illinois - Dick Durbin ( D ) Tammy Duckworth ( D ) D+7
Indiana Mike Braun ( R ) - Todd Young ( R ) R+9
Iowa - Joni Ernst ( R ) Chuck Grassley ( R ) R+3
Kansas - Pat Roberts ( R ) Jerry Moran ( R ) R+13
Kentucky - Mitch McConnell ( R ) Rand Paul ( R ) R+15
Louisiana - Bill Cassidy ( R ) John Kennedy ( R ) R+11
Maine Angus King ( I ) Susan Collins ( R ) - D+3
Maryland Ben Cardin ( D ) - Chris Van Hollen ( D ) D+12
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 such that senators in only one of the classes are up for election in any two-year cycle, rather than having all 100 seats up for election at once. Thus, the 33 Senate seats of class 1 were up for election in 2018, the elections for the 33 seats of class 2 will take place in 2020, and the elections for the 34 seats of class 3 will be held in 2022. The three classes were established by Article I, Section 3, Clause 2 of the U.S. Constitution. The actual division was originally performed by the Senate in May 1789 by lot, with a rule being that a state's two seats had to be in different classes. Whenever a new state subsequently joined the union, its two senate seats were permanently assigned to two different classes by coin toss, while keeping the three classes as close to the same size as possible. A senator's description as junior or senior senator is not related to his or her class. Rather, a state's senior U.S. senator is the one with the greater seniority in the Senate. This is mostly based on length of service.