Federal agencies lose the primary Clean Air Act hook used to regulate CO2 and other greenhouse gases, forcing any new nationwide climate rules to find a different statutory basis or be rebuilt through new rulemaking. Automakers and engine manufacturers no longer face the federal GHG standards that governed 2012–2027 model years, changing compliance planning, certification, and product-mix decisions tied to those requirements. States and litigants are likely to shift enforcement and policy leverage to state-level programs and court challenges, increasing regulatory fragmentation for companies selling across multiple U.S. jurisdictions.