Answer: The American people.
The Preamble of the Constitution states that the people of the United States are the ones who ordain and establish the Constitution to promote the general welfare. Therefore, the American people are the ones who give it power. The text is as follows:
"We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, 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."
