Me. Rev. Stat. tit. 15, § 2309

This compact shall enter into force and become effective and binding upon the states so acting when it has been enacted into law by any 2 states from among the states of Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota and Wisconsin. Thereafter, this compact shall enter into force and become effective and binding as to any other of these states, or any other state upon similar action by such state.