In early use: an advocate of the ballot, a ballotist. Later also: a person who casts a vote in a ballot, a balloter.
Origin
Mid 19th century; earliest use found in The Times. From ballot + -eer. With the sense ‘advocate of the ballot’ compare slightly later ballotist. With the sense ‘person who casts a vote in a ballot’ compare earlier ballotant, balloter.