Me. Const. art. IV, pt. 3, § 21

City council of any city may establish direct initiative and people's veto. The city council of any city may establish the direct initiative and people's veto for the electors of such city in regard to its municipal affairs, provided that the ordinance establishing and providing the method of exercising such direct initiative and people's veto shall not take effect until ratified by vote of a majority of the electors of said city, voting thereon at a municipal election. Provided, however, that the Legislature may at any time provide a uniform method for the exercise of the initiative and referendum in municipal affairs.