Members of the House of Representatives serve two-year terms. Thus, Representatives are up for re-election every two years. Members of the Senate serve six-year terms. Thus, Senators are up for re-election every six years. The President serves a four-year team. Thus, a new President is elected (or the incumbent is re-elected if he or she has not served two terms already) every four years.

Presidential elections always coincide with elections for the House, since two House elections take place within a President's four-year term. Elections for the Senate are staggered through the six-year period, so one-third of Senators are up for re-election every two years.

When elections for the House and Senate occur in a year in which a presidential election is not taking place, it is called a midterm election, since it is midway through a President's four-year term in office.

The House has 435 members. The political party that has 218 members of the House (or the support thereof) is said to be the majority party in the House. The same is true of the Senate, although the number needed for a majority in the 100-member Senate is 51. (The Vice-president can cast a tie-breaking vote if a Senate vote is 50-50 but does not technically have a Senate seat. The Vice-president belongs to the same party as does the President.)

Figures from most past elections show that voter turnout for a midterm election is smaller than that for the same House and Senate races in a presidential election year.