kingmaker

1. (politics) Someone who has strong influence over the choice of a leader.Examples: "Alain, who is proud of choosing the extreme right but still didn't want his real name printed, is one of the 6.4 million [Marine] Le Pen voters who are now the kingmakers in deciding who becomes the next president of France. If the Socialist François Hollande topped the poll, beating the rightwing president, Nicolas Sarkozy, and gaining momentum for returning the left to the presidency for the first time in a generation, it is Le Pen whose high showing and third place now holds the election in the balance."; "However, a National-Act coalition would only have a slim majority, meaning Mr Luxon may need to secure the support of NZ First, whose leader Winston Peters has been kingmaker in previous Labour and National-led coalitions."

2. (by extension, game theory) A player who is unable to win but powerful enough to decide which of the other viable players will eventually win.broadly