Article Five of the United States Constitution
The Founders even put checks and balances on the Legislative Branch's powers.  If Congress gets too powerful, the states can make it call a Constitutional Convention.  If the states get too powerful, ordinary Americans can vote on a Constitutional amendment through ratifying conventions.  As a legal scholar wrote later: "The ratification of the Twenty-First Amendment [by ratifying conventions] might be called one of the most democratic moments in American history. ... Almost all voters in America had the unprecedented opportunity to [support] or condemn a federal constitutional amendment directly.  ... The conventions ratified the choices of the people without fail."pp. 27–28