How many states were needed to ratify the Constitution?

Nine out of the thirteen original states were needed to ratify the Constitution before it could go into effect. Article VII of the Constitution itself set the threshold, declaring that the ratification of the conventions of nine states shall be sufficient for the establishment of this Constitution between the states so ratifying the same. The Founders chose nine deliberately. Requiring unanimous consent, as the Articles of Confederation had for amendments, would have given any single state veto power and made the new union impossible to launch. Requiring a simple majority would have created an unstable foundation. Two-thirds of thirteen states was nine, the same supermajority required for important votes within the Constitution itself, including overriding presidential vetoes and Senate ratification of treaties.

The ratification process began after the Constitutional Convention adjourned on September 17, 1787 and unfolded through state-by-state debates over the following two years. Delaware ratified first on December 7, 1787, followed by Pennsylvania, New Jersey, Georgia, Connecticut, Massachusetts, Maryland, and South Carolina. New Hampshire became the ninth state to ratify on June 21, 1788, achieving the threshold required for the Constitution to take effect. Virginia and New York, two of the most populous and influential states, ratified shortly afterward, on June 25 and July 26, 1788. Without their participation, the new union would have been geographically and politically incoherent.

North Carolina and Rhode Island held out longer. Both initially rejected the Constitution and only ratified after promises that a Bill of Rights would be added by amendment. North Carolina ratified on November 21, 1789, and Rhode Island, the last holdout, on May 29, 1790, more than a year after the new federal government was already operating. The Constitution officially took effect on March 4, 1789, when the new Congress was scheduled to convene, although a quorum did not assemble until April.

Ratification was contested in many states. Federalist supporters of the Constitution, including Alexander Hamilton, James Madison, and John Jay, wrote The Federalist Papers in 1787 and 1788 to defend the document. Anti-Federalist opponents, including Patrick Henry and George Mason, argued for a bill of rights and stronger limits on federal power. The Bill of Rights, ratified in 1791, was the eventual compromise.