The Ninth Circuit affirmed the district court’s denial of motions to intervene as of right filed by the Western States Trucking Association, Construction Industry Air Quality Coalition, American Fuel & Petrochemical Manufacturers, American Petroleum Institute, National Association of Convenience Stores, and various Corn Grower Associations, but reversed the denial of their motions for permissive intervention. The court also reversed the district court’s denial of motions to intervene as of right filed by Alliance for Automotive Innovation, the National Automobile Dealers Association, and the American Free Enterprise Chamber of Commerce, granting them leave to intervene. The court applied the four-part test under Federal Rule of Civil Procedure 24(a)(2) to determine intervention as of right, requiring that a motion be timely, the applicant claim a significantly protectable interest, the disposition impair their ability to protect that interest, and the interest be inadequately represented by existing parties. The court held that Alliance for Automotive Innovation, NADA, and AmFree satisfied these requirements because they possess a legally protectable interest in federal preemption of state vehicle emissions regulations under section 209 of the Clean Air Act. The court found this representation inadequate because the Federal Defendants had not yet raised arguments regarding whether EPA waivers constitute “rules” subject to disapproval under the Congressional Review Act, an argument central to the intervenors’ position. Conversely, the court affirmed the denial for the other groups (WSTA, CIAQC, AFPM, API, NACS, and Corn Growers) because their alleged economic interests were too attenuated from the litigation’s core legal issues regarding the constitutionality of Congressional Review Act resolutions. Regarding permissive intervention under Rule 24(b), the court found the district court abused its discretion by denying motions for AFPM, API, NACS, and the Corn Grower Associations based on an erroneous view that their interests were adequately represented by the Federal Defendants. Because the district court’s decision relied on this incorrect legal analysis, the case was remanded for the district court to re-evaluate whether these groups should be granted permissive intervention. The practical consequence is that Alliance for Automotive Innovation, NADA, and AmFree are immediately permitted to intervene in the underlying litigation, while the other intervenor-defendants must undergo a new hearing on their request for permissive intervention.