ˈstalking ˌhorse noun [countable]
someone or something that hides someone’s true purpose, especially a politician who says they want their leader’s job when the real plan is that another, more important politician should get it

[TahlilGaran] Dictionary of Contemporary English ▲

a stalking horse
1.
a politician who tests the strength of a party's support for its leader by competing for the job of leading the party although they do not really intend to be elected.
He was a stalking horse, intended to undermine what was regarded at the time as a weak leadership.

2.
something that is used to hide someone's real purpose.
It's feared that the talks are just a stalking horse for a much wider deal between the two parties.

[TahlilGaran] English Idioms Dictionary ▲