If there is a conflict between federal law and state law, which law is supreme?
Article VI, Section 2 of the Constitution, often called the "Supremacy Clause," stipulates that the "Constitution, and the Laws of the United States shall be the supreme Law of the Land." What this means, in effect, is that states cannot pass laws that contradict federal laws. If they do, they are null and void. While the balance between federal supremacy and states' rights has tilted in each direction over the years, the Supreme Court has ruled, particularly in the twentieth century, that states cannot pass laws that weaken federal programs or make it more difficult for federal laws to be enforced. Also in the twentieth century, the Supreme Court interpreted the Fourteenth Amendment to maintain that states could not pass laws that contradicted the Bill of Rights. But the foundation for federal supremacy is Article VI, Section 2.
There always is, I think. Here in Canada, provincial politics always face-off with federal. In my view (and I'm not particularly fond of any government), I'd say federal should rule, only because it contains a Constitution (most often), which is the closest these regimes get to de jure freedom. The bad part about provincial politics ruling is that laws change (sometimes radically) from state to state or province to province, and that creates all kinds of problems.