Political parties play an important role in Congress because the houses are organized around parties. Although the Constitution does not mention political parties, they have developed into essential institutions of American politics. Although there were (and are) dozens of political parties, the American political system quickly evolved into a two-party system, which means that two parties have almost always dominated American politics. Since the 1850s, the dominant political parties in the United States have been the Democrats and the Republicans. Each chamber of Congress has a majority party, which holds more than half of the seats, and a minority party, which holds less than half. The parties elect their own leadership, organize for votes, and formulate strategy.
At the start of every congressional session, the parties meet in a caucus, an informal meeting of people with common interests. Caucuses consist of all members interested in a particular issue, and examples include the congressional Black Caucus, the Travel and Tourism Caucus, and Concerned Senators for the Arts. Although caucuses have no formal power, they can be important in formulating bills and rallying support.
The leader of the House of Representatives is the Speaker of the House. The Speaker is elected by the majority party (the Democratic Party Caucus or the Republican Party Caucus, depending on which party controls the House) and sets the schedule for debates and votes on the House floor. The majority party also elects a majority leader, who works closely with the Speaker and the caucus leadership, and several whips, who count votes and connect the leadership to the rank-and-file members. The minority party in the House, meanwhile, elects a minority leader and several whips of its own.
The leadership in the House has a great deal of power over its party because the leaders have the ability to reward and punish members. Members who cooperate with the leadership may be given good committee assignments or even leadership of a committee. Conversely, members who defy leadership may be ostracized by other party members. Party discipline is usually very strong in the House.
According to the Constitution, the vice president of the United States presides over the Senate. In reality, however, the most senior member of the senate—also called the president pro tempore (informally called the president pro tem)—usually presides over the Senate in the vice president’s absence. The president pro tem position is mostly a ceremonial position.