a stalking horse
1. A political candidate who attempts to supplant the current party leader, solely to gauge how much support the incumbent has. He doesn't actually want to be elected—he's just a stalking horse who's trying to see how fractured our party really is.
2. Something that conceals a person's true intentions. I'm afraid that this deal is just a stalking horse for a more nefarious long-term plan.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a stalking horse
1. If you describe an event or an action as a stalking horse, you mean that it is being used to help someone get what they really want at a later date. The development will act as a stalking horse for further exploitation of the surrounding countryside. Limits on union contributions will be a stalking horse to break the relationship between the party and the unions. Note: This expression is usually used to show disapproval.
2. In politics, a stalking horse is someone who stands against the leader of a party to test the strength of any opposition to the leader. They then withdraw in favour of a stronger challenger, if it looks likely that the leader can be defeated. There was even talk of one of them standing for the leadership as a stalking horse for the real contender. Note: You can also use stalking horse before a noun. The notion of a stalking horse challenge at the autumn party conference seemed highly unlikely. Note: Stalking horses were horses that were used by hunters. They were trained to allow their rider to hide behind them, and so get closer to the birds they were hunting.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012