Mass. Gen. Laws ch. 54 § 85A
The state committee of a political party may appoint a person to act as a challenger of voters at any polling place in the commonwealth at a state election, and a city or town committee of such a party, in a city or town in which municipal officers are nominated by primaries or by caucuses of political parties, may appoint a person to act as such challenger at any polling place in such city or town at a municipal election. Such challenger may challenge any voter during the hours that said polling place is open for the purpose of voting; and a statement signed by the chairman of the committee appointing him shall be sufficient evidence of his right so to act. He may be compensated for his services by the political party whose committee appointed him. He shall be assigned by the election officer presiding at the polling place to such position within the polling place as will enable him to see and hear each voter as he offers to vote. Nothing herein contained shall deprive any other person of the right to challenge a voter as provided by law.
Mass. Gen. Laws ch. 54, § 85A