On every even-numbered year, registered voters elect their congressional representatives. Members of the House of Representatives serve two-year terms. Although the U.S. Constitution doesn't limit the number of terms members of Congress can serve, they must run for office again and be re-elected.

Primary Elections

The Republican and Democratic parties dominate the first stage of the election process. In this the U.S. differs from most other representative democracies because party candidates for general election are chosen by voters rather than by party leaders. Each state determines its own procedures for these preliminary contests. In some states the primary election operates like the general election, except that voters only choose among candidates within a single party. Other states hold nominating conventions or caucuses to choose candidates. Voters select their party's candidate to run against the other party's candidate and any independent candidates in the general election.

General Elections

The general election decides which candidates will represent their states or districts in the upcoming term. Unlike the primary elections, voters select among all candidates for office. This includes candidates fielded by the two major parties as well as any independent candidates who have made the ballot. Only the Republican and Democratic parties have primaries, but candidates from other parties can get on the ballot if a state-specified number of registered voters sign a petition to have the candidate's name added. Whichever candidate receives the most votes wins the election.

Direct Election of Senators

According to Article 1, Section 3 of the U.S. Constitution, each state legislature selects that state's two senators. The purpose of this process was to forge stronger ties between federal and state legislatures. This process led to problems, including frequent vacancies resulting in states not being represented in the Senate. In 1913, the 17th Amendment was passed, providing each state's senators would be elected directly by that state's citizens. Candidates for senator must be legal residents of the state who are at least 30 years old and have been U.S. citizens for nine years.

Apportionment of House Seats

The 435 seats in the U.S. House of Representatives are divided among the states according to population distribution. Any legal resident of a state who is at least 25 years old and has been a U.S. citizen for at least seven years is eligible to run for a seat in the House. While the Constitution guarantees each state at least one representative, densely populated states have several districts so representatives can be better attuned to the particular needs of constituents in defined geographic areas. Every 10 years, a census is held to assess the country's population. After each new census, the number of representatives each state is allotted is recalculated to reflect any change in state population since the previous count. States then redraw district lines to ensure each representative serves equivalent numbers of people.