Me. Rev. Stat. tit. 20-A, § 13801

If an administrative unit engages in collective bargaining as required in Title 26, then it shall not enact any ordinance which requires employees to reside within the boundaries of the unit as a condition for employment. A collective bargaining agreement may include a residency requirement for persons not yet employed at the time the agreement becomes effective. If an administrative unit does not engage in collective bargaining as required in Title 26, then any ordinance it enacts which requires employees to reside within the boundaries of the unit shall not apply to persons already employees at the time the regulation becomes effective.