play one's trump card
1. Lit. [in certain card games] to play a card that, according to the rules of the game, outranks certain other cards and is thus able to take any card of another suit. Bob played his trump card and ended the game as the winner.
2. Fig. to use a special trick; to use one's most powerful or effective strategy or device. I won't play my trump card until I have tried everything else. I thought that the whole situation was hopeless until Mary played her trump card and solved the whole problem.