a trump card

a trump card

an advantage that makes you more likely to succeed than other people, especially something that other people do not know about
Usage notes: In card games a trump card is one of a set of cards which have been chosen to have the highest value during the game.
The fact that I had an Italian parent turned out to be my trump card when I applied for the job. Anthea was about to play her trump card - none of the money could be released without her signature.
See also: card, trump
References in classic literature ?
At that moment des Lupeaulx, coming leisurely downstairs to breakfast with the minister, was asking himself whether, before playing a trump card for the husband, it might not be prudent to probe the wife's heart and make sure of a reward for his devotion.
For those in the dark, a trump card can be used franchise to buy back a player they hadn't retained at the start of the auction.
When it comes to betting on big business, The Donald always pulls out a Trump card.