Consisting of 50 states and several territories, the United States is a federal republic with its capital in Washington, D.C. Any citizen over 18 years of age may vote, yet only about half of the people eligible to vote exercise this right. Those who do usually choose between candidates affiliated with the two major political parties, the Democrats and the Republicans. However, there are a number of smaller parties, such as the Libertarian Party, the Green Party, and the recently founded Reform Party. Responsibility for governing is shared between the states and the federal government. The two levels maintain similar structures, as outlined in the U.S. Constitution, which set up a federal government consisting of three branches: legislative, judicial, and executive. The legislature, known as the Congress, is bicameral. The House of Representatives, whose 435 members face election every two years, are apportioned by the populations of the various states. The Senate is made up of 100 members, two from each state, elected for six-year terms. The federal judiciary is comprised of a Supreme Court and a series of lower courts. Executive power is vested in the president who serves a four-year term and can be reelected only once. A variety of agencies operate under the president, making up a vast bureaucracy which carries out the day-to-day tasks of implementing and enforcing laws and regulations. The president is George W. Bush, a Republican, who has held the office since 20 January 2001.