The 2008 United States Senate election in Colorado was held on November 4, 2008. The primary elections were held August 12, 2008. Incumbent Republican U.S. Senator Wayne Allard decided to retire instead of seeking a third term. Democratic nominee Mark Udall won the open seat, the first time in 30 years that a Democrat last held it.
The election featured an open contest because incumbent U.S. Senator Wayne Allard declined to seek re-election. He honored his 1996 pledge to serve no more than two terms in the U.S. Senate and announced that he will retire from his service to the US Senate and not seek a 3rd term, leaving Colorado's Class II Senate seat open. Both parties believed this senate contest would be one of the most competitive senate races during the 2008 election.