Twenty-four states, ten cities, and five counties filed suit in the U.S. Court of Appeals for the D.C. Circuit on Thursday, March 19, challenging the Environmental Protection Agency's repeal of the 2009 endangerment finding — the Obama-era scientific determination that greenhouse gases, including carbon dioxide and methane, threaten public health and welfare. The plaintiffs, all led by Democratic officials, argue the repeal eliminates the legal foundation of virtually all federal climate regulation under the Clean Air Act, including vehicle emissions standards, power plant carbon rules, and industrial pollution limits. The coalition is led by New York Attorney General Letitia James alongside attorneys general from Massachusetts, California, and Connecticut. ABC News and CBS News confirmed the lawsuit and its scope.

The EPA finalized the repeal of the endangerment finding in February 2026. EPA Administrator Lee Zeldin celebrated the action, arguing it would bring the price of a new car or truck down by more than $2,400 and produce economy-wide savings of $1.3 trillion by eliminating what he called regulatory burdens that had inflated vehicle and energy costs without meaningful environmental benefit. The EPA's rationale rested in part on a 2022 Supreme Court ruling in West Virginia v. EPA, which limited how the Clean Air Act can be used to regulate carbon dioxide from power plants, and on the administration's argument that the original 2009 finding relied on contested scientific projections. EPA spokeswoman Brigit Hirsch dismissed the new lawsuit as "not about the law or the merits of any argument" and accused the plaintiffs of being "clearly motivated by politics."

New York AG James said in a statement that the Trump administration had "chosen denial" over protecting Americans from climate threats: "Instead of helping Americans face our new reality, the Trump administration has chosen denial, repealing critical protections that are foundational to the federal government's response to climate change." MPR News and ABC News reported that the endangerment finding had served since 2009 as the legal hook allowing the EPA to regulate six categories of greenhouse gases — carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride — and that its repeal, if upheld, would prevent any future EPA from imposing greenhouse gas rules without new congressional action.

Legal analysts cited by ABC News said the case will almost certainly reach the Supreme Court, which has a 6-3 conservative supermajority. The court's 2022 ruling in West Virginia v. EPA, which limited broad EPA authority under the Clean Air Act, complicates the states' legal position. However, legal scholars noted that the repeal of the endangerment finding goes further than the 2022 ruling contemplated — effectively denying that greenhouse gases are pollutants at all — a position that may be harder to sustain given that the 2007 Supreme Court ruling in Massachusetts v. EPA required the agency to make such a determination if the science supported it. The American Greatness conservative outlet characterized the lawsuit as a predictable overreach by blue-state governors seeking to use litigation to reimpose climate regulations that voters rejected.