The Supremacy Clause puts Federal laws above state laws. Therefore, if the Federal government wants to enforce marijuana laws in Colorado or Washington, it can. Any attempt by those states to interfere would be ruled unconstitutional.

Supremacy Clause

Article VI, Section 2, of the U.S. Constitution is known as the Supremacy Clause because itprovides that the "Constitution, and the Laws of the United States … shall be the supreme Lawof the Land." It means that the federal government, in exercising any of the powers enumeratedin the Constitution, must prevail over any conflicting or inconsistent state exercise of power.
The concept of federal supremacy was developed by Chief Justice John Marshall, who led theSupreme Court from 1801 to 1835. In mcculloch v. maryland, 17 U.S. (4 Wheat.) 316, 4 L. Ed. 579(1819), the Court invalidated a Maryland law that taxed all banks in the state, including a branchof the national bank located at Baltimore. Marshall held that although none of the enumeratedpowers of Congress explicitly authorized the incorporation of the national bank, the Necessary and Proper Clause provided the basis for Congress's action. Having established that theexercise of authority was proper, Marshall concluded that "the government of the Union, thoughlimited in its power, is supreme within its sphere of action."
After the Civil War, the Supreme Court was more supportive of States' Rights and used theTenth Amendment, which provides that the powers not delegated to the federal government arereserved to the states or to the people, to justify its position. It was not until the 1930s that theCourt shifted its position and invoked the Supremacy Clause to give the federal governmentbroad national power. The federal government cannot involuntarily be subjected to the laws ofany state.
The Supremacy Clause also requires state legislatures to take into account policies adopted bythe federal government. Two issues arise when State Action is in apparent conflict with federallaw. The first is whether the congressional action falls within the powers granted to Congress. IfCongress exceeded its authority, the congressional act is invalid and, despite the SupremacyClause, has no priority over state action. The second issue is whether Congress intended itspolicy to supersede state policy. Congress often acts without intent to preempt state policymaking or with an intent to preempt state policy on a limited set of issues. Congress may intendstate and federal policies to coexist.
Some federal legislation preempts state law, however, usually because Congress believes itslaw should be supreme for reasons of national uniformity. For example, the National LaborRelations Act of 1935 (Wagner Act) (29 U.S.C.A. § 151 et seq.) preempts most state lawdealing with labor unions and labor-management relations.
In Pennsylvania v. Nelson, 350 U.S. 497, 76 S. Ct. 477, 100 L. Ed. 640 (1956), the SupremeCourt developed criteria for assessing whether federal law preempts state action when Congresshas not specifically stated its intent. These criteria include whether the scheme of federalregulations is "so pervasive as to make the inference that Congress left no room for the Statesto supplement it," whether the federal interest "is so dominant that the federal system [must] beassumed to preclude enforcement of state laws on the same subject," or whether theenforcement of a state law "presents a serious danger of conflict with the administration of thefederal program."
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