WASHINGTON—The Supreme Court ruled unanimously Wednesday that states may not impose excessive fines, extending a bedrock constitutional protection but potentially jeopardizing asset-forfeiture programs that help fund police operations with property seized from criminal suspects.

“For good reason, the protection against excessive fines has been a constant shield throughout Anglo-American history: Exorbitant tolls undermine other constitutional liberties,” Justice Ruth Bader Ginsburg wrote for the court, warning that tyrants have used such fines to punish or silence political enemies.

More recently, the proliferation of fines has alarmed criminal-justice advocates, who contend they fall disproportionately on the poor and distort police priorities.

An array of organizations filed briefs backing the constitutional argument for curbing excessive fines, from the liberal-leaning American Civil Liberties Union and the NAACP Legal Defense and Educational Fund to the U.S. Chamber of Commerce and the Foundation for Moral Law on the right.

“Today’s ruling should go a long way to curtailing what is often called ‘policing for profit’—where police and prosecutors employ forfeiture to take someone’s property then sell it, and keep the profits to fund their departments,” said Wesley Hottot, an attorney with the libertarian Institute for Justice who successfully argued the case in November.