The U.S. Supreme Court is set to hear a significant case that could impact the Environmental Protection Agency's (EPA) authority to enforce clean air regulations, particularly those involving state plan components. Attorneys have indicated that the outcome may limit the EPA's ability to defend its rules in the D.C. Circuit. A coalition of Republican state attorneys general has filed a petition arguing that an appellate court should not consider late justifications for the EPA’s traveling ozone rule. Meanwhile, a group of Democratic-led state governments is asserting that the Clean Air Act does not require the D.C. Circuit to reconsider the EPA's 2023 'Good Neighbor' emissions regulation, which has been contested by two steel manufacturers. This ongoing litigation highlights the contentious nature of environmental regulation in the U.S. and its implications for businesses.