United States Senate is one of the two houses of the legislature (Congress) of the United States, established in 1789 under the Constitution. Each state elects two senators for six-year terms. The terms of about one-third of the Senate membership expire every two years. Senators are elected directly by voters of each state. The Senate shares with the House of Representatives responsibility for all lawmaking within the United States. For an act of Congress to be valid, both houses must approve an identical document.