In addition to his work with the Farm Bureau, he was general manager of the Wyoming Rural Electrification Administration. After five years in the Wyoming House, Thomas won a special election to replace Dick Cheney as Wyoming's lone member of the United States House of Representatives; he was re-elected to that seat in 1990 and 1992. In 1994, he ran for the United States Senate and won, defeating popular Democratic Governor Mike Sullivan by 20 percentage points. He was elected second term in 2000 with a 74 percent majority, one of the largest margins in Wyoming election history. In the 2006 election he was opposed by Democratic engineer Dale Groutage. Thomas was re-elected to a third term with 70 percent of the ballots even as Democratic Governor Dave Freudenthal was also winning with the same 70 percent margin.