Environmental groups and even some of their opponents say that whatever the court decides, EPA still will be able to move forward with broader plans to set emission standards for greenhouse gases for new and existing power plants.
But a court ruling against EPA almost undoubtedly would be used to challenge every step of the agency's effort to deal with climate change, said Jacob Hollinger, a partner with the McDermott Will and Emery law firm in New York and a former EPA lawyer.
"Will they be successful? We don't know yet," Hollinger said. "But it would be an important victory in a political sense and, potentially, a practical sense."
Republicans have objected strenuously to the administration's decision to push ahead with the regulations after Congress failed to pass climate legislation, and after the administration of President George W. Bush resisted such steps. Both sides agree that it would have been better to deal with climate change through legislation than regulation.
In 2012, a three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit concluded that the EPA was "unambiguously correct" in using existing federal law to address global warming.
Monday's case, for which the court has expanded argument time to 90 minutes from the usual 60, stems from the high court's 2007 ruling in Massachusetts v. EPA, which said the agency has the authority under the Clean Air Act to limit emissions of greenhouse gases from vehicles.
Two years later, with Obama in office, the EPA concluded that the release of carbon dioxide and other heat-trapping gases endangered human health and welfare. The administration used that finding to extend its regulatory reach beyond automobiles and develop national standards for large stationary sources. Of those, electric plants are the largest source of emissions.
The administration has proposed first-time national standards for new power plants and expects to propose regulations for existing plants this summer. It will then move on to other large stationary sources such as factories.
In the meantime, the only way EPA can compel companies to address global warming pollution is through a permitting program that requires them to analyze the best available technologies to reduce carbon dioxide, the chief greenhouse gas.
The utility industry, the U.S. Chamber of Commerce and 13 states led by Texas are asking the court to rule that the EPA overstepped its authority by trying to regulate greenhouse gas emissions through the permitting program.
The EPA's actions "represent one of the boldest seizures of legislative authority by an executive agency in history," Peter Keisler, representing the American Chemistry Council among two dozen manufacturing and industry groups that want the court to throw out the rule, said in court papers.
When the Supreme Court considered the appeals in October, the justices declined requests to consider overruling the court's 2007 decision, review the EPA's conclusion about the health effects of greenhouse gas emissions or question limits on vehicle emissions.
Instead, the court focused on the permitting program, which EPA has said it would apply for the time being only to the largest emitters of greenhouse gases.
The relatively narrow question framed by the court has led environmental advocates to minimize the case's significance.
"Twice, the Supreme Court has affirmed the EPA's authority to regulate climate pollution," said Vickie Patton, general counsel for the Environmental Defense Fund. Patton was referring to the 2007 decision and the court's 2011 decision that said only EPA, not states and conservation groups, could seek cuts in power plant emissions.