The U.S. Supreme Court has stripped federal agencies of authority over millions of acres of wetlands, weakening a bedrock environmental law enacted a half-century ago to cleanse the country’s badly polluted waters.

A 5-4 majority significantly expanded the ability of farmers, homebuilders and other developers to dig up or fill wetlands near rivers, lakes and streams, finding the government had long overreached in limiting such activities.

The ruling Thursday may nullify key parts of a rule the Biden administration imposed in December, which two federal judges already had blocked from being enforced in 26 states. It’s the latest turn in a decades-old struggle by courts and regulators to determine which waters are subject to protection under the Clean Water Act.

Some experts say the battle over wetlands now may shift to states, with red and blue states writing laws that take dramatically different approaches.

The high court’s decision follows one in 2022 curtailing federal power to reduce carbon emissions from power plants and indicates a willingness by the court’s emboldened conservatives to limit environmental laws and agency powers.

“This is one of the saddest chapters in the 50-year history of the Clean Water Act,” said Jim Murphy, an attorney with the National Wildlife Federation.