According to both the Declaration of Independence and the Constitution of the United States, where does the government get its power?

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.