WordNet (r) 3.0 (2006):

Kingmaker
    n 1: 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)
         [syn: Warwick, Earl of Warwick, Richard Neville,
         Kingmaker]
    2: an important person who can bring leaders to power through
       the exercise of political influence; "the Earl of Warwick was
       the first kingmaker"

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "kingmaker":
   Machiavellian, Rasputin, Svengali, VIP, access, bad influence,
   behind-the-scenes operator, big wheel, court, eminence grise,
   finagler, five-percenter, friend at court, gamesman,
   good influence, gray eminence, heavyweight, hidden hand, influence,
   influence peddler, influencer, ingroup, key, lobby, lobbyist,
   logroller, lords of creation, machinator, man of influence,
   manipulator, open sesame, operator, pork-barrel politician,
   powers that be, pressure group, sinister influence,
   special interests, special-interest group, strategist,
   the Establishment, very important person, wheeler-dealer,
   wire-puller