Most CO US Senators have served no more than 2 terms
Class 2
Udall-D 2009-2015 defeated
Allard-R 1997-2003,2003-2009 retired
Brown-R 1991-1997 retired
Armstrong-R 1979-1985,1985-1991 retired
Haskell-D 1973-1979 defeated
Allott-R 1955-1961,1961-1967,1967-1973 defeated
Johnston-D 1937-1943,1943-1949,1949-1955 retired
Costigan-D 1931-1937 retired
Phipps-R 1919-1925, 1925-1931 retired.
Class 3
Salazar-D 2005-2009 resigned
Campbell-R 1993-1999,1999-2005 retired
Wirth-D 1987-1993 retired
Hart-D 1975-1981,1981-1987 retired
Dominick-R 1963-1969,1969-1975 defeated
Carroll-D 1957-1963 defeated
Millikin-R 1941-1945,1945-1951,1951-1957 retired
Adams-D 1933-1939,1939-1941 died
Current Class 2 CO US Senator Gardner-R (2015-present)loses re-election in 2020 to Hickenlooper-D or Perlmutter-D. Hickenlooper-D or Perlmutter-D step down after 1 term in 2026- Jared Polis-D becomes US Senator after serving 2 terms as Governor.
Current Class 3 CO US Senator Bennet-D ( 2009-2011,2011-2017,2017-2023)is likely to retire in 2022 or 2028.
Neguse-D or Crow-D replaces Bennet-D in 2022 or 2028.