Imagine that Alice is the President of Arstotzka. Alice has a lot of enemies but she's generally an upstanding President so her critics have a hard time building a campaign against her. Instead, the critics are patiently waiting in the shadows until Alice makes a minor mistake - perhaps she misspeaks at a conference or her car driver skips a stop sign. The critics then blow this incident out of proportion and use it to smear Alice before the next election.
What's a word or expression specifically describing the act/strategy of waiting for a small transgression to happen?