Federal power
Since the 1930s the Supreme Court of the United States has interpreted the Commerce Clause of the Constitution of the United States in an expansive way that has dramatically expanded the scope of federal power.[citation needed] For example, Congress can regulate railway traffic across state lines, but it may also regulate rail traffic solely within a state, based on the theory that wholly intrastate traffic can still have an impact on interstate commerce.
Another source of Congressional power is its "spending power"—the ability of Congress to impose uniform taxes across the nation and then distribute the resulting revenue back to the states (subject to conditions set by Congress). A classic example of this is the system of "federal-aid highways", which includes the Interstate Highway System. The system is mandated and largely funded by the federal government, but also serves the interests of the states. By threatening to withhold federal highway funds, Congress has been able to pressure state legislatures to pass a variety of laws. Although some object that this infringes on states' rights, the Supreme Court has upheld the practice as a permissible use of the Constitution's Spending Clause.