stalking horse

(dated, hunting) A horse used as cover by a hunter stalking game.

(idiomatic, politics) A candidate put forward to serve a hidden, ulterior purpose in a political campaign, such as testing the field for another potential candidate by gauging voter sentiment or covertly helping another candidate by attracting voters away from a third candidate.

(idiomatic, by extension) A person, thing, or expedient used in a deceptive manner, to achieve some hidden purpose.