The Constitution's grants of power to the federal government are anchored in several important clauses. The Commerce Clause, found in Article I, Section 8, grants Congress the power to regulate commerce among the states, with foreign nations, and with Indian tribes. Since Gibbons v. Ogden (1824), this clause has been broadly interpreted, making it one of the most significant sources of federal regulatory authority. The Necessary and Proper Clause, also called the Elastic Clause, empowers Congress to make all laws necessary and proper for carrying out its enumerated powers. McCulloch v. Maryland (1819) established that this clause authorizes implied powers, allowing Congress to legislate in areas not explicitly mentioned so long as the legislation is reasonably designed to achieve legitimate constitutional ends. Together, these clauses have enabled Congress to address problems the framers could not have anticipated.
The Supremacy Clause in Article VI establishes that the Constitution, federal laws, and treaties constitute the supreme law of the land. This means that when federal and state laws conflict, federal law prevails. The doctrine of preemption, derived from the Supremacy Clause, describes how federal law can override or displace state law. Preemption may be express, when Congress explicitly states its intent to preempt state law, or implied, when federal regulation is so pervasive that it occupies the field, or when state law stands as an obstacle to federal objectives. Even when Congress has not legislated, the Dormant Commerce Clause doctrine inferred from the Commerce Clause restricts states from passing laws that unduly burden or discriminate against interstate commerce, ensuring a national economic union.
Other clauses structure the relationship between states and between government and private parties. The Full Faith and Credit Clause requires each state to recognize the public acts, records, and judicial proceedings of every other state, ensuring that court judgments and marriages valid in one state are generally honored elsewhere. The Privileges and Immunities Clause of Article IV prevents states from discriminating against citizens of other states regarding fundamental rights, while a separate Privileges or Immunities Clause in the Fourteenth Amendment was intended to protect fundamental rights of national citizenship, though its scope was narrowed by the Slaughter-House Cases (1873). The Ex Post Facto Clauses in Article I prohibit Congress and the states from passing retroactive criminal laws that punish actions legal when committed, increase punishment after the fact, or alter evidentiary rules to make conviction easier. The Takings Clause of the Fifth Amendment requires just compensation when the government takes private property for public use, applying to both physical takings through eminent domain and regulatory takings that deprive owners of all economically viable use of their property.
Several amendments have also reshaped the structure of government and federal powers. The Sixteenth Amendment (1913) authorized a federal income tax without apportionment among the states, overturning an earlier Supreme Court ruling and enabling the modern federal revenue system. The Seventeenth Amendment (1913) provided for the direct election of United States Senators by the people. The Eighteenth Amendment (1919) established Prohibition, which the Twenty-First Amendment (1933) repealed, the only amendment to be repealed. The Twenty-Second Amendment (1951) limits presidents to two terms in office, and the Twenty-Fifth Amendment (1967) addresses presidential succession and disability. The Constitution also provides for the impeachment of federal officials, with the House of Representatives bringing charges by majority vote and the Senate conducting trials requiring a two-thirds vote for conviction on grounds of treason, bribery, or other high crimes and misdemeanors. The Electoral College, established by Article II and refined by the Twelfth Amendment, elects the President and Vice President through 538 electors, with 270 electoral votes needed to win. The War Powers Resolution (1973) limits the President's ability to commit armed forces without congressional approval, and although executive privilege is recognized, the Supreme Court held in United States v. Nixon (1974) that it is not absolute and cannot shield evidence in a criminal prosecution. The Court's decisions in Baker v. Carr (1962), which established that legislative redistricting cases are justiciable, and Bush v. Gore (2000), which halted a Florida recount under the Equal Protection Clause, illustrate how constitutional principles shape the political process. In Korematsu v. United States (1944), the Court controversially upheld Japanese American internment during World War II, a decision later described as "gravely wrong" by the Court itself.