First elected to the House of Representatives as a Republican in 1984, he was re-elected twice before running for the Senate in 1990, winning the open seat and assuming it early when incumbent Gordon J. Humphrey resigned. He was re-elected in 1996 and then entered the Republican primary for the 2000 presidential election. After failing to gain traction, he withdrew before the primaries began and joined the Taxpayers' Party, seeking their nomination instead. He then changed parties again, becoming an Independent before dropping out of the presidential race altogether. He then re-joined the Republican Party after the Chairmanship of the Senate Committee on Environment and Public Works became open, which he then assumed.
Smith ran for re-election in 2002 but lost the Republican primary to Congressman John E. Sununu, who won the general election. After his defeat he moved to Florida and briefly ran for the Senate from there in 2004 and in 2010, but dropped out early on in both cases after faring poorly in polls of the Republican primary. He subsequently returned to New Hampshire, where he declared his candidacy for the Republican nomination in the 2014 Senate election, but was defeated in the Republican primary by former U.S. Senator from Massachusetts Scott Brown.
