South Dakota Senator Tom Daschle,the minority leader in the Senate, was defeated by former Republican Representative John Thune in Tuesday's elections, dealing a heavy blow to Democrats' efforts to regain control of the Congress.

Daschle became the first party leader to lose a race for reelection in more than half a century.

With votes in 99 percent of the precincts in South Dakota counted early Wednesday, Daschle trailed Thune by more than 8,000 votes in the sparsely settled state.

Republicans appeared certain to maintain control over the House and expand their majority in the Senate.

Republicans were assured of 53 seats in the 100-member Senate, and were expected to retain control of the 435-seat House.