The Constitution provides for a union of states, now numbering 50, each with its own constitution, republican form of government, and reserved powers, within a federal system. The national government is responsible for external affairs and has concurrent powers with states, commonwealths, and self-governing territories over domestic matters. The chief of state is the president of the United States and the seat of government is the District of Columbia, which has limited home rule and no voting representation in the national legislature.

The Constitution establishes three separate branches of government: the legislative, executive, and judicial. Each branch has its own area of authority. These areas overlap, making it necessary for the three branches to share in, and compete for, the power to govern effectively. Each branch has some constitutional authority that it can use to impede the functioning of the other branches, creating a system of checks and balances. The purpose of this somewhat cumbersome machinery of government, as intended by the framers of the Constitution, is to prevent the concentration of power in a small group of politicians, which could lead to tyranny.

Since the adoption of the Constitution, the national government has increased its functions in economic and social matters and has shared more responsibilities with the states.

The American president typically has a greater range of functions than prime ministers in parliamentary governments because the president serves as ceremonial chief of state as well as head of government. Unlike most presidents in other nations, the American president is also the head of his or her party, an important legislative leader, and the chief executive.

The Constitution makes the president commander in chief of the U.S. armed forces. The president defends the nation against invasion or attack and may order American armed forces into combat. The president's authority to deploy forces on his or her own initiative is regulated by Congress under Article I, Section 8, which reserves to Congress the power to declare war, and under provisions of the War Powers Resolution of 1973.

The president's diplomatic powers include negotiation and ratification of treaties, with the consent of two-thirds of the Senate; the appointment of ambassadors to foreign nations, also with the consent of the Senate; and the reception of foreign ambassadors. The president negotiates, on his or her own authority, executive agreements with leaders of other nations.

By law the president prepares an executive budget and an economic report, which are submitted to Congress each year. The president submits requests for legislation, the most important of which usually regard taxation and other economic and military matters. The president also exercises executive authority over the various government departments and agencies.

All legislative powers granted by the Constitution in Article I are exercised by the Congress of the United States. Congress consists of two houses, the Senate and the House of Representatives. The Senate contains 100 senators, two representing each state—a provision of the Constitution not subject to amendment. The 435 members of the House are elected by the different states on the basis of their population at the most recent U.S. census. California has the largest number of representatives, 52; several states, such as Delaware and Vermont, have only 1. Representatives serve two-year terms, and senators six-year terms. Every two years all 435 members of the House are elected, and one-third of the senators. In presidential election years, about 45 percent of eligible adults vote for members of Congress; in other election years, only about 35 percent vote.

The Senate and House are organized by the majority party in each chamber, which chooses the presiding officer, the majority leader, and the chairpersons of each committee. Through much of American history the party controlling the White House did not control both houses of Congress. This situation, known as divided government, can lead to reduced output of legislation and an increase in presidential vetoes of bills passed by Congress. Unlike the chief executives of parliamentary systems in other countries, the U.S. president neither resigns nor calls for new elections, even when majorities in Congress reject the president's programs.

Congress has extensive powers in domestic affairs, including the power to tax, borrow money and pay debts, coin money and regulate its value, and regulate commerce among the states. Congress helps to establish and oversees the departments and agencies of the executive branch; it also establishes the lower federal courts and determines their jurisdiction. Congress has the power to declare war, raise and maintain the armed forces, establish tariffs, and regulate commerce with foreign nations.

A bill is passed by Congress by majority vote of those present in each chamber; it is then sent to the president. The president may sign the bill, to indicate approval, or allow the bill to become law without signing it; or may veto the bill and return it to Congress, giving reasons for this action. The president's veto can be overridden by a two-thirds vote of the members of Congress voting in each chamber.

Each house of Congress has some distinct powers. Revenue measures must originate in the House of Representatives. The House, with a majority vote, can initiate proceedings to impeach (charge with misconduct) the president. If the Electoral College cannot produce a majority to elect a president, the House chooses one of the top three contenders. If both the president and the vice president die, are incapacitated, or are removed from office, the Speaker of the House becomes president.

The Senate advises and consents to presidential treaties and to nominations for major executive officials, ambassadors, justices of the Supreme Court, and federal judges. The Senate tries all impeachments, with a two-thirds vote necessary to convict. In the event of a deadlock in the Electoral College, the Senate chooses the vice president from the top two contenders. The president pro tempore of the Senate comes after the Speaker of the House in the line of succession to the presidency.

The federal courts perform two constitutional functions. First, they interpret the meaning of laws and administrative regulations; this is known as statutory construction. Second, the courts determine whether any law passed by Congress or state legislatures, or any administrative action taken by the national or state executive branches, violates the U.S. Constitution; this is known as judicial review. Federal courts can declare null and void laws or actions, at the national and state levels, that violate the Constitution. This power of judicial review exists in a few other nations, but in none is it so significant in resolving important issues or in checking and balancing branches of government.

The nine justices of the Supreme Court and the other federal judges are nominated by the president with the advice and consent of the Senate. The president, in making district court nominations, usually follows the recommendations of senators from the president's party. All federal judges and justices of the Supreme Court serve on good behavior for life. They may be removed from office only through the process of impeachment, which has been used fewer than 20 times, and never successfully against a Supreme Court justice.

Decisions of the Supreme Court that involve the statutory construction of laws may be overturned by Congress. Decisions involving judicial review may be checked and balanced in either of two ways. The president and Senate may deliberately fill vacancies on the Supreme Court with new justices who can be expected to overturn the decision; or the Constitution can be amended, as was the case after the Supreme Court ruled income tax unconstitutional.

The U.S. Constitution provides for a federal system, with those powers not exercised by the national government retained by the states. States are denied the power to conduct foreign relations, enter into treaties or alliances, or lay tariffs. They may not coin currency, levy taxes on interstate commerce, or prevent the movement of persons across their borders. States may cooperate with one another through creation of interstate compacts, which require the approval of Congress. These often involve water resources, navigation, pollution control, or port development.

The national government and states are closely linked in an administrative system of cooperative federalism. This includes categorical grant programs, in which the national government establishes operating standards and pays up to 90 percent of the cost of programs administered by the states; block grants for general purposes such as education or community development; and revenue sharing, whereby the national government distributes money to states and localities each year.

The major functions of the states include qualified control of voter eligibility requirements; administration of state and national elections; supervision of municipal and county government; promotion and regulation of commerce, industry, and agriculture; and maintenance of highways, prisons, hospitals, and mental-health facilities. The states also support extensive systems of higher education. They share with local units of government the responsibility for welfare, medical care for indigents, employment services, and other social services.

Almost all states are divided into territorial units called counties. In 1992 the United States had 3043 counties. Louisiana is divided into 64 parishes, which are similar to counties. Alaska has no counties as such; much of the state is organized into 16 boroughs. In a number of states, such as Connecticut, counties have virtually no governmental function. In several states, notably Virginia, one or more cities are independent of any county organization and thus constitute primary divisions of the state. In relatively heavily populated areas, communities are organized into a total of 19,296 municipalities, which include cities, towns, villages, and boroughs. Municipalities generally provide basic services, including police, sanitation, and fire protection. Education at the elementary and secondary levels usually is supervised by school boards, which share authority over finance, curriculum, and teacher certification with state government; the United States had 14,556 school districts in 1992, down from 108,579 in 1942. Also important are so-called special districts, which are independent, limited-purpose local government units dealing with water supply, flood control, fire protection, community development, housing, and other matters. The United States had 33,131 special districts in 1992.

In the late 20th century the Democrats were split into two major factions. The northern Democrats as a rule favored national action to solve social problems, emphasized government regulation of the economy, and favored strong action to aid minorities. The southern Democrats were more conservative in fiscal, economic regulation, and social matters.

Federal, state, and local grants provide income assistance for the blind, disabled, and elderly poor, and assistance to poor families with dependent children. School lunch programs for needy children and a food stamp program for poor families are also provided.