In the case of Engine Manufacturers Association and Western States Petroleum Association v. South Coast Air Quality Management District et al., 2003, the Supreme Court ruled in favor of the plaintiffs, stating that local air quality management districts do not have the authority to enforce regulations on emissions from new vehicles or engines. The South Coast Air Quality Management District (SCAQMD) had previously enacted fleet rules requiring operators of certain vehicle fleets within their jurisdiction to purchase lower-emitting vehicles when adding or replacing units in their fleets. However, these were found to be pre-empted by federal law under section 209(a) of the Clean Air Act which prohibits states and local governments from setting emission standards for new motor vehicles or engines. The court held that SCAQMD's fleet rules amounted to de facto emission standards as they pressured manufacturers into creating cleaner-running vehicles.

In the dissenting opinion of this case, Justice Souter argued that the South Coast Air Quality Management District's fleet rules did not fall under preemption by federal law. He contended that these rules were purchasing requirements rather than performance standards and thus should be considered separately from emission control regulations. The Clean Air Act, he pointed out, expressly allows states to adopt stricter standards for emissions if they choose to do so. Moreover, he suggested that interpreting the act in a way that would preempt local governments' ability to make their own purchasing decisions could lead to an overly broad interpretation of federal power over state and local affairs. In his view, such an interpretation would undermine principles of federalism and potentially disrupt other areas where state or local governments have traditionally had autonomy.