The U.S. Supreme Court is considering the question of who should have the power to order utility companies to cut their greenhouse gas emissions.
Six states, New York City and three conservation groups filed the lawsuit to force five companies, including American Electric Power Co., to cut their emissions of the gases that contribute to climate change.
The White House has sided with the utility companies, urging the high court to throw out the case because the Environmental Protection Agency is already in the process of setting emission standards.
"Congress set up the EPA to promulgate standards for emissions," Justice Ruth Bader Ginsburg told the lawyer representing the six suing states. "The relief you're seeking seems to me to set up a district judge, who does not have the resources, the expertise, as a kind of super EPA."
The case is the second climate change dispute at the Supreme Court in four years.
In 2007, the court declared that carbon dioxide and other greenhouse gases are air pollutants and the EPA has the authority to regulate those emissions from new cars and trucks, as well as power plants.
A decision in the latest case is expected by late June.