The Clean Air Act, a federal statute, establishes national air pollution standards for fleet vehicles such as buses, taxicabs, and trucks. The South Coast Air Quality Management District (South Coast) is a political entity of the state of California. South Coast establishes air pollution standards for the Los Angeles, California, metropolitan area. South Coast enacted fleet rules that prohibited the purchase or lease by public and private fleet operators of vehicles that do not meet stringent air pollution standards set by South Coast. South Coast’s fleet emission standards are more stringent than those set by the federal Clean Air Act. The Engine Manufacturers Association (Association), a trade association that represents manufacturers and sellers of vehicles, sued South Coast, claiming that South Coast’s fleet rules are preempted by the federal Clean Air Act. The U.S. District Court and the U.S. Court of Appeals upheld South Coast’s fleet rules. The Association appealed to the U.S. Supreme Court. Are South Coast’s fleet rules preempted by the federal Clean Air Act?