Me. Rev. Stat. tit. 22, § 752

This compact shall become effective when enacted into law by any 2 or more of the states of Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont. Thereafter it shall become effective with respect to any other aforementioned state upon its enacting this compact into law. Any state not mentioned in this Article which is contiguous to any party state may become a party to this compact by enacting the same.