The primary author of the U.S. Constitution is James Madison, who is often referred to as the "Father of the Constitution." Madison played a crucial role at the Constitutional Convention in 1787, advocating for a strong federal government and articulating the principles of checks and balances that would define the new government structure. He meticulously documented the debates, and his contributions laid the foundation for the final document. Additionally, Madison later authored many of the Federalist Papers, which were instrumental in promoting the ratification of the Constitution, emphasizing the need for a robust central government while safeguarding individual rights.