Both the Declaration of Independence and the Constitution were heavily influenced by the Enlightenment and especially by the theories of John Locke concerning the source of power or legitimacy for governments.
Hereditary monarchies had drawn their authority from legitimacy of descent, in which monarchs passed their power down to their children. A second theory, popular in theocracies such as Egypt or ancient Mesopotamia and revived in early modern Europe, was the "divine right of kings," which argued that kings drew their authority from God. Another notion was that raw power itself constituted a form of authority or legitimacy ("might makes right").
The radical Enlightenment notion found in these two documents was that a government draws its power and legitimacy from the people and governs with the consent of the governed. Thus the Constitution begins with the phrase "we the people," who are seen collectively as the holders of power. Public servants such as presidents, congressmen, and civil servants do not have power themselves but are chosen by the people to do certain necessary administrative tasks.
Both the Declaration of Independence and the Constitution of the United States proclaim the power of the government comes from the people. In the preamble to the Constitution, the opening words indicate that this is where the power originates:
"We the people of the United States, in order to form a more perfect union, establish justice, insure domestic tranquility, provide for the common defense, promote the general welfare, and secure the blessings of liberty to ourselves and our posterity, do ordain and establish this Constitution for the United States of America."
The Declaration of Independence makes the same claim. In fact, it also makes it within the opening words:
"The unanimous Declaration of the thirteen united States of America, When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation."
The two documents present the idea of democracy in different ways. The Constitution directly states that the people have the power to create the governing rules, while the Declaration states the people have the power to pull away from an existing governing body in the face of tyranny and injustice.