The US constitution says: "This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall

The US constitution says: "This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the Supreme law of the land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding." That means that when the federal government has jurisdiction, laws passed at the federal level preempt state laws that contradict them. I don't know where the EPA rules gain their constitutional authority, my guess is the commerce clause of the constitution. But fundamentally, what this means is that when the federal government passes a law, state laws that contradict it, are, according to the Supreme Court, "without effect". I'd imagine that occurs in a positive sense and negative sense - when something like a car that gets 40 miles to a gallon or a THC-filled vape becomes legal at the federal level, states aren't allowed to decide otherwise, without delegation from the government. NCLB is different because while the federal government can make laws, it can't force the states to implement those laws. While there could be a federal agency - like the classroom police - to enforce NCLB, it's easier to just bribe the states until they agree to comply. https://en.wikipedia.org/wiki/Federal_preemption https://en.wikipedia.org/wiki/Federal_preemption