The ninth-circuit affirmed the 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, while reversing the district court’s denial of permissive intervention motions for the latter four groups and remanding for further proceedings. The court applied the four-part test under Federal Rule of Civil Procedure 24(a)(2) to determine intervention as of right, requiring a timely motion, a significantly protectable interest, a showing that the disposition may impair the ability to protect that interest, and a demonstration that the interest is inadequately represented by existing parties. The court held that Auto Innovators, NADA, and AmFree satisfied this test because they possess a legally protectable right under section 209 of the Clean Air Act to sell federally approved vehicles, an interest that would be impaired if the Plaintiff States succeeded in their challenge to the Congressional Review Act resolutions disapproving EPA waivers. Furthermore, these groups demonstrated inadequate representation by Federal Defendants, who had not yet raised arguments regarding whether EPA waivers constitute “rules” under the Administrative Procedure Act and the CRA. Conversely, the court found that the economic interests asserted by WSTA, CIAQC, AFPM, API, NACS, and the Corn Grower Associations were too attenuated from the litigation’s core issues to constitute a significantly protectable interest. Regarding permissive intervention, the court determined the district court abused its discretion by denying motions for AFPM, API, NACS, and the Corn Grower Associations based on an erroneous view that Federal Defendants adequately represented their interests; because Federal Defendants may not advance all arguments these organizations wish to make, the district court must re-evaluate whether permissive intervention is appropriate. The practical consequence is that Auto Innovators, NADA, and AmFree are granted leave to intervene as of right in the underlying litigation concerning California’s zero-emission vehicle regulations. The motions to intervene as of right by WSTA, CIAQC, AFPM, API, NACS, and the Corn Grower Associations remain denied, but AFPM, API, NACS, and the Corn Grower Associations are remanded to seek permissive intervention in the district court.