Kenneth Robert Buck (born February 16, 1959) is an American lawyer and politician who represents Colorado's 4th congressional district in the United States House of Representatives as a Republican. From March 30, 2019, to March 27, 2021, Buck served as chair of the Colorado Republican Party, having replaced Jeff Hays. Formerly the District Attorney for Weld County, Colorado, Buck ran unsuccessfully for U.S. Senate in 2010, narrowly losing to Democrat Michael Bennet. In Congress, Buck has emerged as one of the foremost proponents of antitrust enforcement in the Republican Party.