A candidate put forward to divide the Opposition or to mask the true candidate WordNet 3.0 & Open English WordNet

A stalking horse is a figure used to test a concept or mount a challenge on behalf of an anonymous third party. Wikipedia

a figure used to test a concept or mount a challenge on behalf of an anonymous third party Wikidata

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. Wiktionary