U.S. Supreme Court justices have debated putting new limits on the Environmental Protection Agency’s power to tackle greenhouse-gas emissions, as they considered a case that threatens to undercut U.S. President Joe Biden’s climate agenda.

Hearing two hours of arguments in Washington, the justices gave a mixed reception to Republican-led states and coal companies seeking to bar the EPA from issuing a sweeping plan to reduce carbon emissions from power plants. The Biden administration is defending the agency’s authority, with backing from power companies and environmental groups.

Justice Samuel Alito questioned the administration’s assertion that it can broadly push electricity generation away from high-emission sources, as long as it doesn’t impose unreasonable costs or threaten grid reliability.