Me. Rev. Stat. tit. 21-A, § 334

A candidate for nomination by primary election must file a primary petition and consent under sections 335 and 336. The candidate must be enrolled, on or before March 15th, in the party named in the petition and must be eligible to file a petition as a candidate for nomination by primary election under section 144, subsection 3. The registrar in the candidate's municipality of residence must certify to that fact on a form designed by the Secretary of State.