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.