The Constitution rests on several foundational principles that distribute power and prevent its concentration. Separation of powers divides governmental authority among three distinct branches: the legislative branch makes the laws, the executive branch enforces them, and the judicial branch interprets them. This division ensures that no single branch can dominate the others. Complementing this division is the system of checks and balances, by which each branch restrains the others. The President may veto legislation, but Congress can override the veto with a two-thirds vote; the Senate confirms judicial and executive appointments; the House can impeach officials and the Senate can convict them; and the judiciary can declare laws and executive actions unconstitutional through the power of judicial review.
Federalism further structures American government by dividing power between a central national government and regional state governments. The Constitution grants certain powers to the federal government, called enumerated powers, which are specifically listed in Article I, Section 8 and include the power to tax, regulate commerce, coin money, establish post offices, and declare war. Powers not delegated to the federal government and not prohibited to the states are reserved to the states or the people under the Tenth Amendment. Examples of reserved powers include regulating intrastate commerce, conducting elections, and exercising general police powers. Some powers, called concurrent powers, are shared by both levels of government, such as the power to tax, build roads, and establish courts. When federal and state laws conflict, the Supremacy Clause gives precedence to federal law.
The principle of judicial review, although not explicitly stated in the Constitution, has become a cornerstone of American constitutional law. It was established in Marbury v. Madison (1803), in which Chief Justice John Marshall asserted the Supreme Court's authority to declare acts of Congress unconstitutional. Marshall's opinion reasoned that the Constitution is the supreme law and that courts must interpret it when applying it to cases, making it the Court's duty to strike down laws that conflict with constitutional provisions. This decision transformed the judiciary into a co-equal branch capable of checking the legislative and executive branches and is widely regarded as the most important ruling in American constitutional law.
In the decades following Marbury, the Supreme Court defined the contours of federal power in cases like McCulloch v. Maryland (1819) and Gibbons v. Ogden (1824). In McCulloch, the Court upheld Congress's creation of a national bank, finding that Congress possesses implied powers under the Necessary and Proper Clause to carry out its enumerated responsibilities. The decision also established that states cannot tax federal institutions, with Marshall famously observing that "the power to tax involves the power to destroy." In Gibbons, the Court broadly interpreted the Commerce Clause, ruling that Congress's power to regulate interstate commerce includes navigation and broadly extends to commercial activity that crosses state lines. Together, these decisions expanded federal authority and laid the groundwork for the modern regulatory state.