From Longman Dictionary of Contemporary Englishtrump/best/strongest cardtrump/best/strongest cardADVANTAGEsomething that gives you a big advantage in a particular situation The promise of tax cuts proved, as always, to be the Republican Party’s trump card. → card
Examples from the Corpus
trump/best/strongest card• We had beaten him, but he played a final trump card.• In the struggle for development, every economy has certain advantages or trump cards.• That night, though, our sincerity was our trump card.• The citizens of Hebron, by contrast, hold all the trump cards.• That was why Gorbachev wanted to negotiate-and that is why, in my opinion, President Reagan was holding the trump card.• Parents must recognize that if a child does not want to do homework, the child holds the trump card.• This was one of the trump cards of News International in its dispute with the print workers in 1986-87.• And perhaps it was time to play the trump card up his sleeve. 