Definitions for kingmakerˈkɪŋˌmeɪ kər
Random House Webster's College Dictionary
one who has sufficient political power to influence the choice of candidates for public office.
Origin of kingmaker:
Warwick, Earl of Warwick, Richard Neville, Kingmaker(noun)
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)
an important person who can bring leaders to power through the exercise of political influence
"the Earl of Warwick was the first kingmaker"
Someone who helps making a leader a force to reckon with, without having any desire for the position themselves.