A high-stakes dispute over the Environmental Protection Agency’s power to regulate greenhouse gases from stationary sources divided the U.S. Supreme Court on Monday along classic liberal-conservative lines.

During more than 90 minutes of oral argument in Utility Air Regulatory Group v. EPA, the court’s more liberal members appeared ready to defer to the EPA in interpreting an unclear legislative mandate to regulate air pollutants.

“Why shouldn’t we leave it to the agency” when so many interpretations of the EPA’s power are possible, Justice Sonia Sotomayor asked Peter Keisler of Sidley Austin, who represented five sets of private plaintiffs challenging the EPA. In earlier briefs, those plaintiffs themselves disagreed over how to interpret the Clean Air Act, Sotomayor pointed out.

Conservatives on the high court seemed skeptical, suggesting that the EPA was trying to rewrite acts of Congress to expand the agency’s reach into even minor sources of greenhouse-gas pollution.

Justice Anthony Kennedy and Chief Justice John Roberts Jr. may be swing votes in the case. In their questioning of Keisler, both suggested the court has some obligation not to ignore the court’s 2007 precedent Massachusetts v. EPA, which said the EPA did have authority to regulate greenhouse gases emitted by motor vehicles.

Contact Tony Mauro at tmauro@alm.com. On Twitter: @Tonymauro.