kingmaker (n.)

also king-maker, 1590s, originally in reference to Richard Nevil, Earl of Warwick (d. 1471), credited with elevating Edward IV and after restoring Henry VI.

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Definitions of kingmaker from WordNet
kingmaker (n.)
an important person who can bring leaders to power through the exercise of political influence;
the Earl of Warwick was the first kingmaker
Kingmaker (n.)
English statesman; during the War of the Roses he fought first for the house of York and secured the throne for Edward IV and then changed sides to fight for the house of Lancaster and secured the throne for Henry VI (1428-1471);
Synonyms: Warwick / Earl of Warwick / Richard Neville