The primary author of the Constitution of the United States was James Madison, who is often referred to as the "Father of the Constitution." Madison played a crucial role in drafting and promoting the document during the Constitutional Convention in 1787. His influence is evident in many of the Constitution's foundational principles, including the system of checks and balances and the separation of powers. He also authored the Federalist Papers, which argued for ratification and explained the framework of the new government. Madison's contributions were instrumental in establishing the legal foundation of the United States.