Me. Const. art. IV, pt. 3, § 19

Any measure referred to the people and approved by a majority of the votes given thereon shall, unless a later date is specified in said measure, take effect and become a law in 30 days after the Governor has made public proclamation of the result of the vote on said measure, which the Governor shall do within 10 days after the vote thereon has been canvassed and determined; provided, however, that any such measure which entails expenditure in an amount in excess of available and unappropriated state funds shall remain inoperative until 45 days after the next convening of the Legislature in regular session, unless the measure provides for raising new revenues adequate for its operation. The veto power of the Governor shall not extend to any measure approved by vote of the people, and any measure initiated by the people and passed by the Legislature without change, if vetoed by the Governor and if the veto is sustained by the Legislature shall be referred to the people to be voted on at the next general election. The Legislature may enact measures expressly conditioned upon the people's ratification by a referendum vote.