The United States of America, as a federation of now 50 states each with its own constitution and legal system still enjoying a large degree of governmental autonomy within the national legal framework, presents a strikingly mixed picture regarding the use of direct democracy--the submission of proposed governmental action to a popular vote--in law- and constitution-making processes. At the national level, direct democracy has never been used for either type of enactment. At the state and local level, however, its use dates back to colonial times and has been increasing gradually (though still not universal) ever since. Since the mid-19th century, every new state admitted to the union has been required by Congress to submit its initial constitution to popular vote, and all but one of the state constitutions now require a popular vote to ratify all constitutional changes proposed by the most commonly used methods.
William B. Fisch, Constitutional Referendum in the United States of America, 54 Am. J. Comp. L. 485 (2006)