The determining factor in how the court rules could be Justice Anthony Kennedy. But it was unclear from his questioning which side he will take. He told Verrilli, “I can’t find a single precedent that strongly supports your position.”

Verrilli responded that EPA has had to act quickly because of the urgency of anticipated effects of climate change.

The court is expected to issue a ruling in the case, Utility Air Regulatory Group v. EPA, by the end of June.