The Supremacy Clause (Article 6, Clause two) declares that the Constitution is the "supreme law of the land". It establishes that federal laws rule over state laws. This ensures that there will be no disagreement between the state and the national government over who has more authority. Federalism is a system of government where the power is shared between the state and national government, but this does not mean that they are equal. The national government will be "supreme".
