The U.S. Supreme Court on Thursday significantly curtailed the authority of the Environmental Protection Agency to regulate emissions from power plants, prompting Democrats to froth at the mouth because the court said Congress must act if lawmakers are concerned about climate change.

In a 6-3 decision in West Virginia v. Environmental Protection Agency, the court said that the EPA lacks broad authority to regulate greenhouse gas emissions from power plants under the Clean Air Act.

"Congress did not grant EPA ... the authority to devise emissions caps based on the generations shifting approach the Agency took in the Clean Power Plan," the court said.

The case concerned an Obama-era EPA regulation known as the Clean Power Plan, which created guidelines for states to limit carbon dioxide emissions from power plants, and its Trump-era replacement, known as the Affordable Clean Energy rule. The Trump administration repealed the Clean Power Plan after arguing it was illegal and introduced its own rule, known as the ACE rule, as a replacement. But some states and environmental groups sued, claiming Trump's rule did not go far enough and that the EPA had erred by issuing the ACE rule instead of following through with the Clean Power Plan.