The major political document signed by the Founding Fathers in 1787 is the United States Constitution. This document established the framework for the federal government and defined the powers and responsibilities of its three branches: the executive, legislative, and judicial. Additionally, it established a system of checks and balances to prevent any one branch from becoming too powerful. The Constitution also included a Bill of Rights, which guaranteed fundamental rights and liberties to citizens. The signing of the Constitution marked a pivotal moment in American history, as it created a lasting blueprint for democracy and governance in the United States.