Final answer:
The Federalists promised to add a Bill of Rights to the Constitution to protect individual liberties, which was fundamental in securing the ratification of the Constitution, especially from pivotal states like Virginia and New York.
Step-by-step explanation:
The Federalists had to make a crucial promise to ensure the ratification of the Constitution. They agreed to add a Bill of Rights, which would enumerate and protect individual liberties, alleviating fears of a tyrannical central government. Without this pledge, the support of key states such as Virginia and New York might not have been secured, and the Constitution could have failed to become the law of the land. This promise was instrumental in bringing about the necessary compromise between Federalists, who advocated for a strong central government, and Anti-Federalists, who were concerned about the potential abuse of federal power.