trump card

1. Games. A card in the trump suit, held in reserve for winning a trick.
2. A key resource to be used at an opportune moment; a trump: "[They] seem determined to use the agreement as a trump card to obtain ... all the advantages they feel they deserve" (Christian Science Monitor).

noun

Something, especially something held in reserve, that gives one a decisive advantage: trump. Informal clincher. Idioms: ace in the hole. See help/harm/harmless, win/lose/recovery.

A key resource to gain an advantage at the opportune moment, as in That surprise witness was the defense's trump card, or She played her trump card, announcing that the Senator would speak. This expression transfers the trump card of games such as bridge, which can win over a card of another suit, to other kinds of advantage. [Early 1800s]

The noun has one meaning:

Meaning #1: a playing card in the suit that has been declared trumps
  Synonym: trump