The primary architect of the United States Constitution is often regarded as James Madison. As a key figure in the Constitutional Convention of 1787, Madison played a crucial role in drafting and promoting the Constitution. His contributions included advocating for a strong central government while balancing the interests of both large and small states. He also co-authored the Federalist Papers, which argued for the ratification of the Constitution and explained its principles. Madison's vision and political philosophy shaped the foundational framework of the U.S. government, making him a pivotal figure in American history.