It implies cleverly using something specific to gain a desirable outcome in competitive or strategic situations. During the debate, he played the underdog card to win public support.
Pro Tip 2/3
Emotional Appeal
Often invokes empathy or moral support, suggesting the player is looking for a particular emotional response. Whenever she's criticized, she plays the victim card to divert attention.
Pro Tip 3/3
Strategic Timing
Timing is key; using the phrase at the right moment can make it much more effective. Right before the vote, he played the patriotism card to sway the undecided.