When the people, acting as sovereign states, ratified the U.S. Constitution, they deliberately created a federal government with very limited powers. Each power was specifically enumerated in this Constitution. Most of the debate during the ratification process was about how to be sure the states' new creation, the federal government, wouldn't usurp powers that weren't delegated to it by the states and thus meddle in the responsibilities of the states and in the lives of the people.
The three branches of government and other checks and balances were a part of the answer. Most importantly, the states retained their power to be the final judge of what's constitutional and what isn't. If a federal law isn't made in pursuance of the powers delegated in the Constitution as determined by a state, that state has every power, right and duty to make the unconstitutional federal law null and void in that state.
The process of nullification has been used many times. State action trumps unconstitutional federal action.
Clark Staiger, North Enid