Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 51, § 20

If there is a vacancy in the board of registrars caused by death, resignation or retirement or if a member of the board of registrars is unable to perform the duties of his office, or is, at the time of any meeting of said board, absent from the city or town, the mayor or selectmen may, upon the request in writing of a majority of the remaining members of the board, appoint in writing some person to fill such vacancy temporarily, who is of the same political party as the member whose position he is appointed to fill. Such temporary registrar shall perform the duties and be subject to the requirements and penalties provided by law for a registrar of voters.