stalking horse

Short definition from Wiktionary

Noun

1. 1
   A horse used as cover by a hunter stalking game.
2. 2
   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.
3. 3
   A person, thing, or expedient used in a deceptive manner, to achieve some hidden purpose.
4. 4
   Alternative form of stalking horse.