deliver a/the knockout punch

1. In boxing and similar sports, to strike someone with a blow that renders them unconscious or technically disqualifies them from continuing. The newcomer delivered the knockout punch in the first 30 seconds of the second round. She was behind for nearly the whole match, but she delivered an incredible knockout punch that laid her opponent out flat. I know he's the reigning champ, but, wow—I didn't expect him to deliver the knockout punch that fast!

2. By extension, to do something that completely and decisively ensures the defeat or downfall of someone or something. The Supreme Court's decision delivered a knockout punch to our hopes of overturning the legislation. Their company was already struggling to survive, but the arrival of a new big-box retailer next door delivered the knockout punch. I expect this major scandal will deliver the knockout punch to that candidate's chances of getting elected.