Why did james madison promise to add a bill of rights to the constitution?

The original Constitution (proposed in 1787 in Philadelphia) contained very few individual rights guarantees. In the ratification debate, Anti-Federalists (later Jeffersonians) opposed to the Constitution, complained that the new system threatened liberties, and said that if the delegates had cared about protecting individual rights, they would have included provisions. In the end Federalists agreed to take up the matter of a series of amendments, to be called the Bill of Rights.