In 1989, Dick Cheney, who occupied Wyoming's only seat in the House of Representatives, resigned to become Secretary of Defense. Thomas became the Republican candidate to succeed Cheney and won the April 1989 special election. He was re-elected in 1990 and 1992, and in 1994 he ran for and won the Senate seat being vacated by fellow conservative Republican Malcolm Wallop of Sheridan in northeastern Wyoming. He was re-elected in 2000 and 2006, having easily beaten Democratic candidates in both elections with 70 percent of the vote.