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.