dark horse
n. an unknown entrant into a contest; a surprise candidate for political office.The party is hoping that a dark horse will appear before the election.You'd be surprised at how eagerly people will vote for a dark horse.
throw one's hat in the ring
tr. to indicate that one is to be a contestant or a candidate.The con claimed he needed parole so he could throw his hat in the ring for the mayoral election.I won't throw my hat into the ring until the last minute.