What is the primary source of the law that is at issue here?
Suppose the California legislature passes a law that severely restricts carbon dioxide emissions from automobiles in that state. A group of automobile manufacturers files suit against the state of California to prevent the enforcement of the law. The automakers claim that a federal law already sets fuel economy standards nationwide and that fuel economy standards are essentially the same as carbon dioxide emission standards. According to the automobile manufacturers, it is unfair to allow California to impose more stringent regulations than those set by the federal law.