Thomas Jefferson's key contributions to the Declaration of Independence include eloquently articulating the principles of individual rights and the concept of government deriving its power from the consent of the governed. His famous phrase, "life, liberty, and the pursuit of happiness," encapsulated these ideals, advocating for the inherent rights of individuals. Jefferson's vision for a democratic society emphasized the importance of freedom and the responsibility of citizens in governance. His authorship of this critical document positioned him as a foundational figure in shaping American political ideology and the nation’s quest for independence.