Back at Montpelier, still undecided on a profession, Madison soon embraced the patriot cause, and state and local politics absorbed much of his time. In 1775 he served on the Orange County committee of safety; the next year at the Virginia convention, which, besides advocating various Revolutionary steps, framed the Virginia constitution; in 1776-77 in the House of Delegates; and in 1778-80 in the Council of State. His ill health precluded any military service.

In 1780 Madison was chosen to represent Virginia in the Continental Congress (1780-83 and 1786-88). Although originally the youngest delegate, he played a major role in the deliberations of that body. Meantime, in the years 1784-86, he had again sat in the Virginia House of Delegates. He was a guiding force behind the Mount Vernon Conference (1785), attended the Annapolis Convention (1786), and was otherwise highly instrumental in the convening of the Constitutional Convention in 1787. He had also written extensively about deficiencies in the Articles of Confederation.

Madison was clearly the preeminent figure at the convention. Some of the delegates favored an authoritarian central government; others, retention of state sovereignty; and most occupied positions in the middle of the two extremes. Madison, who was rarely absent and whose Virginia Plan was in large part the basis of the Constitution, tirelessly advocated a strong government, though many of his proposals were rejected. Despite his poor speaking capabilities, he took the floor more than 150 times, third only after Gouverneur Morris and James Wilson. Madison was also a member of numerous committees, the most important of which were those on postponed matters and style. His journal of the convention is the best single record of the event. He also played a key part in guiding the Constitution through the Continental Congress.

Playing a lead in the ratification process in Virginia, too, Madison defended the document against such powerful opponents as Patrick Henry, George Mason, and Richard Henry Lee. In New York, where Madison was serving in the Continental Congress, he collaborated with Alexander Hamilton and John Jay in a series of essays that in 1787-88 appeared in the newspapers and were soon published in book form as The Federalist (1788). This set of essays is a classic of political theory and a lucid exposition of the republican principles that dominated the framing of the Constitution.

In the U.S. House of Representatives (1789-97), Madison helped frame and ensure passage of the Bill of Rights. He also assisted in organizing the executive department and creating a system of federal taxation. As leaders of the opposition to Hamilton's policies, he and Jefferson founded the Democratic-Republican Party.

In 1794 Madison married a vivacious widow who was 16 years his junior, Dolley Payne Todd, who had a son; they were to raise no children of their own. Madison spent the period 1797-1801 in semiretirement, but in 1798 he wrote the Virginia Resolutions, which attacked the Alien and Sedition Acts. While he served as Secretary of State (1801-9), his wife often served as President Jefferson's hostess.

In 1809 Madison succeeded Jefferson. Like the first three Presidents, Madison was enmeshed in the ramifications of European wars. Diplomacy had failed to prevent the seizure of U.S. ships, goods, and men on the high seas, and a depression wracked the country. Madison continued to apply diplomatic techniques and economic sanctions, eventually effective to some degree against France. But continued British interference with shipping, as well as other grievances, led to the War of 1812.

The war, for which the young nation was ill prepared, ended in stalemate in December 1814 when the inconclusive Treaty of Ghent which nearly restored prewar conditions, was signed. But, thanks mainly to Andrew Jackson's spectacular victory at the Battle of New Orleans (Chalmette) in January 1815, most Americans believed they had won. Twice tested, independence had survived, and an ebullient nationalism marked Madison's last years in office, during which period the Democratic-Republicans held virtually uncontested sway.

In retirement after his second term, Madison managed Montpelier but continued to be active in public affairs. He devoted long hours to editing his journal of the Constitutional Convention, which the government was to publish 4 years after his death. He served as co-chairman of the Virginia constitutional convention of 1829-30 and as rector of the University of Virginia during the period 1826-36. Writing newspaper articles defending the administration of Monroe, he also acted as his foreign policy adviser.

Madison spoke out, too, against the emerging sectional controversy that threatened the existence of the Union. Although a slaveholder all his life, he was active during his later years in the American Colonization Society, whose mission was the resettlement of slaves in Africa.

Madison died at the age of 85 in 1836, survived by his wife and stepson.