The Supreme Court has declined to revive a contentious Missouri law that prohibited local law enforcement from assisting federal officials in the enforcement of federal gun regulations. This decision comes after the Justice Department challenged the 2021 law, known as the Second Amendment Preservation Act, arguing that it violated the Supremacy Clause of the Constitution.

The Missouri law imposed penalties of up to $50,000 for aiding in the enforcement of federal laws deemed possibly unconstitutional by the state's General Assembly. These included requirements for the registration of certain dangerous weapons and the maintenance of records by firearms dealers. However, exceptions were made for state agencies to assist in the enforcement of federal laws that have state equivalents, particularly those related to felony crimes.

Earlier this year, a federal judge in Missouri ruled against the law, siding with the Biden administration. The judge's decision blocked the law from taking effect, and subsequent appeals failed to overturn that ruling, prompting Missouri Attorney General Andrew Bailey to seek intervention from the Supreme Court. However, the court ultimately rejected the state's petition.

Justice Clarence Thomas expressed a dissenting opinion, stating his support for allowing the law to go into effect. Justice Neil Gorsuch, with the agreement of Justice Samuel Alito, issued a brief statement aligning with the court's order "under the present circumstances."

Missouri Attorney General Andrew Bailey argued in his emergency application to the Supreme Court that the federal government lacked the necessary legal standing to challenge the state law. Bailey emphasized that the authority to restrict state resources for federal enforcement falls within the purview of a state's Tenth Amendment rights.

The federal judge who initially ruled against the law, Brian Wimes, stated that it was "invalidated as unconstitutional in its entirety." Wimes noted that state and local law enforcement officials in Missouri maintained the ability to participate in federal firearm crime investigations and task forces without fear of penalties imposed by the Second Amendment Preservation Act.

Opposing the emergency request, Solicitor General Elizabeth Prelogar deemed the Missouri law "patently unconstitutional" in her court filings. She emphasized that the Supremacy Clause of the Constitution prohibits states from nullifying or obstructing the enforcement of federal laws.

The implementation of the Missouri law earlier this year raised concerns among local authorities, causing potential complications for federal agents operating in the state.