Explain how the Founding Fathers established the concept of legitimacy of power?

Answer 1

It was originally based on "consent of the governed," but that clearly is no longer the case and arguably hasn't been for most of U.S. history. Lysander Spooner argued that the Constitution, even if one assumes its legitimacy, lost all contractual effect when the men who signed it died.

See "No Treason: The Constitution of No Authority":

http://www.lysanderspooner.org/notreason.htm