Thomas Jefferson played a critical role in shaping the founding principles of the U.S. as the principal author of the Declaration of Independence. His articulation of the ideals of liberty, equality, and the pursuit of happiness laid the philosophical groundwork for American democracy. Jefferson's vision for a nation founded on individual rights and freedoms influenced the structure of the U.S. government and its emphasis on separation of powers. Additionally, as the third President of the United States, he further helped shape policy and promote values aligning with democratic principles and civic responsibility.