T he Constitution establishes a "system of dual sovereignty between the States and the Federal Government," with each state having its "own government," "endowed with all the functions essential to separate and independent existence." As the Supreme Court has recognized, states "possess sovereignty concurrent with that of the Federal Government, subject only to limitations imposed by the Supremacy Clause," the provision of the Constitution that makes federal law the "supreme Law of the Land" and prohibits states from contravening lawful enactments of Congress. Although the Supremacy Clause grants Congress a degree of authority to "impose its will on the States," the federal government may not exceed "the powers granted it under the Constitution." The Constitution only endows the federal government with a "limited" and "defined" set of enumerated powers, while reserving most other powers to the states. As a consequence, "States retain broad autonomy in structuring their governments and pursuing legislative objectives." The various principles that delineate the proper boundaries between the powers of the federal and state governments are collectively known as "federalism," a doctrine based on the Framers' conclusion that allocating "powers between the National Government and the States enhances freedom, first by protecting the integrity of the governments themselves, and second by protecting the people, from whom all governmental powers are derived." Federalism has informed modern understandings of the limits on Congress's authority in a number of areas. For instance, the Supreme Court has identified limits on Congress's enumerated powers, such as its power to regulate interstate commerce under Article I, Section 8 of the Constitution. The Court has also recognized other federalism-based doctrines that constrain Congress's power, such as the anti-commandeering doctrine—that is, the prohibition against the national government demanding that a state use its own governmental system to implement federal commands. Because the various jurisprudential developments that limit the power of the federal government are important considerations whenever Congress legislates, federalism is a "closely watched" and "ever-present" issue for Congress. This report thus provides a broad overview of the various legal doctrines that inform the boundaries of Congress's authority vis-à-vis the states under the Constitution. The report begins by addressing several general principles that undergird modern legal debates over federalism. The report then discusses "internal" limitations on Congress's exercise of several of its enumerated powers, including its spending and commerce powers, its authority to enact certain types of legislation to implement international treaties, its enforcement authority under the Civil War Amendments, and its powers under the Necessary and Proper Clause. The report concludes by discussing several "external" federalism-based limitations on Congress's powers, such as the anti-commandeering, anti-coercion, state sovereign immunity, and equal sovereignty doctrines. The Constitution imposes two broad limitations on the powers of Congress. First, the concept of enumerated powers creates what is often referred to as an "internal limit" on Congress's powers —that is, Congress's powers are restricted by and to the terms of their express grant. To illustrate, as one commentator has noted, while Congress can exercise its power over federal enclaves to prescribe a fire code for the District of Columbia, the terms of that power are internally limited by the terms of the Enclave Clause; thus, Congress could not, for example, invoke the Enclave Clause to write a fire code for the State of Delaware. Second, beyond the internal limits on Congress's powers, the Constitution also imposes "external" constraints on congressional action—that is, affirmative prohibitions found elsewhere in the text or structure of the document. In other words, even if Congress is acting consistent with the terms of an enumerated power—say, by prescribing a fire code for the District of Columbia pursuant to the Enclave Clause—external limits on that power would necessarily prohibit Congress from inserting certain terms into that code—such as by withholding fire protection from individuals of a particular race in contravention of the equal protection component of the Fifth Amendment's Due Process Clause. As relevant here, the Supreme Court's federalism jurisprudence sets forth both internal and external constraints on Congress's power vis-à-vis the states. The internal federalism-based limitations on Congress's powers are "embedded within the clauses that grant enumerated powers to the national government," such as Article I's Commerce Clause or the Fourteenth Amendment's Enforcement Clause. The external federalism-based constraints on Congress's powers generally arise from the Tenth Amendment or structural features of the Constitution, which recognize that, as part of the "constitutional design," "both the National and State Governments have elements of sovereignty the other is bound to respect." Before discussing the various internal and external federalism-based limits on Congress's powers, it is important, as a threshold matter, to note how these limitations are enforced. In the modern era, there have been two "competing conceptions of federalism." Exemplifying one viewpoint is the Court's 1985 decision in Garcia v. San Antonio Metropolitan Transit Authority , which held that the Constitution does not insulate state governments from the reach of generally applicable laws enacted pursuant to Congress' power under the Commerce Clause. In so holding, the Court concluded that the "principal and basic limit" on Congress's powers vis-à-vis the states is "the built-in restraints that our system provides through state participation in federal governmental action." Put another way, the Garcia Court concluded that the "political processes" (i.e., Congress's and the President's discretion), and not the Court, would be the primary means to enforce the federalism-based limits on Congress's powers. However, the political process conception of federalism embraced in Garcia has largely been supplanted in more recent years with the view that the judiciary must safeguard state governments from federal overreach. For instance, in 1995, the Court in United States v. Lopez struck down a federal law that forbade possessing a gun within 1,000 feet of a school, holding that the law exceeded Congress's powers under the Commerce Clause. In concluding as such, the Lopez Court described the federalism-based limitations on Congress's commerce power as limits that '"the Court has ample power' to enforce." Nonetheless, while the modern Court has recognized that the judiciary can police the limits of Congress's powers vis-à-vis the states, as Justice Kennedy noted in his concurrence in Lopez , the political branches continue to have a central role in recognizing the limits on Congress's powers to enact legislation that potentially intrude on areas reserved to state governments. Specifically, Justice Kennedy noted the following: Whatever the judicial role, it is axiomatic that Congress does have substantial discretion and control over the federal balance. For these reasons, it would be mistaken and mischievous for the political branches to forget that the sworn obligation to preserve and protect the Constitution in maintaining the federal balance is their own in the first and primary instance. . . . The political branches of the Government must fulfill this grave constitutional obligation if democratic liberty and the federalism that secures it are to endure. As a result, the federalism-based limits on Congress's power discussed in the remainder of this report are not only significant considerations for Congress to ensure that a court does not invalidate legislation on federalism grounds; in addition, the doctrine of federalism may be an important background principle for legislators to consider to help ensure that the political process respects state sovereign interests. The Constitution confers certain enumerated powers upon Congress, many of which are beyond the scope of this report. However, some of Congress's powers—namely, its powers under the Spending Clause, the Commerce Clause, the Treaty Power, the Civil War Amendments, the Necessary and Proper Clause, and—are particularly relevant to defining the appropriate allocation of power between the federal government and the states. The following subsections of the report accordingly discuss internal limitations the Constitution imposes on Congress's exercise of those particular powers. One source of congressional authority to enact legislation that may potentially impact the powers of the states is the Spending Clause, which empowers Congress to "lay and collect Taxes, Duties, Imposts, and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States." The breadth of Congress's Spending Clause authority, along with the question of whether the Clause imposed any internal limitations on Congress's authority, were the subject of much debate for the first 150 years of the nation's history. The debate over the meaning of the Spending Clause centered on two Framers of the Constitution: James Madison and Alexander Hamilton. Madison, fearing the potential scope of Congress's power, argued for a more limited view of Congress's power under the Spending Clause, contending that Congress's power to spend money for the "general welfare" was restricted to expenditures under the enumerated powers in Article I, Section 8 of the Constitution. In contrast, Hamilton viewed the Spending Clause as conferring an independent power for Congress to raise and spend money to promote the national welfare. The Supreme Court resolved the debate over the scope of the spending power in the 1936 case of United States v. Butler , in which the Court concluded that the Hamiltonian interpretation of the Clause was "the correct one." Specifically, the Court held that the "confines" on the Spending Clause are "not limited by the direct grants of legislative power found in" the remainder of Article I, Section 8 of the Constitution. The Court then reaffirmed Butler 's holding regarding the spending power in two subsequent decisions that upheld the Social Security Act of 1935's unemployment insurance and pension system. The Court has not reversed course on its adoption of the Hamiltonian view of the Spending Clause, with the result that Congress may attain certain "objectives not thought to be within Article I's 'enumerated legislative fields,' . . . through the use of the spending power."