According to Article 1 of the U.S. Constitution (1787), Congress consists of two houses, the Senate (upper house) and the House of Representatives (lower house).
Congress approves the federal budget, imposes uniform taxes and duties throughout the country, sets the national debt ceiling, regulates foreign and domestic commerce, and oversees the activities of the executive branch. Congress has the power to declare war and form the armed forces (the president may use the armed forces without a declaration of war for up to 60 days). The House of Representatives also has the right to elect the president of the USA if during presidential elections none of the candidates receives more than half of the electoral votes (in the history of the USA there have been two such cases: in 1801 and 1824).
The most important powers of the Senate are to approve international treaties and presidential appointments to top government positions. The Senate also has the power, on the recommendation of the House of Representatives, to impeach the president.
Elections are held on the first Tuesday after the first Monday in November of an even-numbered year. In a leap year, they coincide with presidential elections and are called national elections; in a non-leap year they are interim because they fall in the middle of the incumbent’s term in office and serve as an indicator of the level of popular support for the party whose representative occupies the White House.
The U.S. has historically had a strong two-party system. Since the 1850s, both houses of Congress and state legislatures have been controlled by the Democratic and Republican parties. Other political forces do not play a prominent role in the life of the country.
As a rule, candidates go through the procedures of preliminary party elections (primaries), in some states they are chosen by party congresses.
Members of Congress are elected by direct majority vote. Election procedures are established by state law. In most states, elections are held in one round, using the relative majority principle (to win, it is enough to get more votes than the competitor and not necessarily more than half). However, in some states (e.g. Louisiana and Georgia) an absolute majority is required to win, which sometimes leads to a 2nd round. The constitution does not limit the number of terms for which members of both houses may be elected.
Elections are financed in part from the state budget, personal funds of candidates, and donations. A 2010 Supreme Court decision, upheld in 2014, lifted restrictions on the flow of corporate money to party coffers and candidate funds, allowing virtually unlimited and in some cases anonymous sponsorship.
All U.S. citizens who are at least 18 years old and reside in the U.S. for a specified period of time (its length is determined by state law) have the right to vote actively.