The Collaborative International Dictionary of English v.0.48:

Supporter \Sup*port"er\, n.
   1. One who, or that which, supports; as, oxygen is a
      supporter of life.
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            The sockets and supporters of flowers are figured.
                                                  --Bacon.
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            The saints have a . . . supporter in all their
            miseries.                             --South.
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   2. Especially, an adherent; one who sustains, advocates, and
      defends; as, the supporter of a party, faction, or
      candidate.
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   3. (Shipbuilding) A knee placed under the cathead.
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   4. (Her.) A figure, sometimes of a man, but commonly of some
      animal, placed on either side of an escutcheon, and
      exterior to it. Usually, both supporters of an escutcheon
      are similar figures.
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   5. (Med.) A broad band or truss for supporting the abdomen or
      some other part or organ.
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WordNet (r) 3.0 (2006):

supporter
    n 1: a person who backs a politician or a team etc.; "all their
         supporters came out for the game"; "they are friends of the
         library" [syn: supporter, protagonist, champion,
         admirer, booster, friend]
    2: someone who supports or champions something [syn: patron,
       sponsor, supporter]
    3: a person who contributes to the fulfillment of a need or
       furtherance of an effort or purpose; "my invaluable
       assistant"; "they hired additional help to finish the work"
       [syn: assistant, helper, help, supporter]
    4: a band (usually elastic) worn around the leg to hold up a
       stocking (or around the arm to hold up a sleeve) [syn:
       garter, supporter]
    5: a support for the genitals worn by men engaging in strenuous
       exercise [syn: athletic supporter, supporter,
       suspensor, jockstrap, jock]

Moby Thesaurus II by Grady Ward, 1.0:

186 Moby Thesaurus words for "supporter":
   Maecenas, Santa Claus, abettor, acquaintance, adherent, admirer,
   advocate, aficionado, aid, almoner, almsgiver, alpenstock,
   alter ego, angel, apologist, appendage, arm, assignor, assistant,
   athletic supporter, attendant, awarder, back, backbone, backer,
   backing, bandeau, bearer, best friend, bestower, bosom friend, bra,
   brace, bracer, braces, bracket, brassiere, brother, buff, buttress,
   cane, carrier, casual acquaintance, cavaliere servente, cervix,
   champion, cheerful giver, close acquaintance, close friend, cohort,
   conferrer, confidant, confidante, consignor, contributor, corset,
   courtier, crook, crutch, dangler, defender, dependence, dependent,
   devotee, disciple, donator, donor, encourager, endorser,
   enthusiast, exponent, expounder, fairy godmother, familiar, fan,
   favorer, fellow, fellow creature, fellowman, feoffor, financer,
   flunky, follower, following, foundation garment, friend,
   friend at court, fulcrum, funder, galluses, girdle, giver, grantor,
   grubstaker, guard, guy, guywire, hanger, hanger-on, helper,
   henchman, homme de cour, imparter, inseparable friend, intimate,
   jock, jockstrap, lady bountiful, lover, mainstay, maintainer, mast,
   meal ticket, neck, neighbor, other self, pair of suspenders,
   paladin, paranymph, parasite, partisan, patron, patroness,
   philanthropist, pickup, presenter, promoter, prop, proponent,
   protagonist, protector, public, pursuer, pursuivant, reinforce,
   reinforcement, reinforcer, reliance, repository, rest,
   resting place, rigging, satellite, second, seconder, sectary,
   settler, shadow, shoulder, shroud, sider, spine, sponsor, sprit,
   staff, staker, stalwart, standby, standing rigging, stave, stay,
   stick, stiffener, stooge, strengthener, subscriber, successor,
   sugar daddy, support, suspender, suspenders, sustainer,
   sympathizer, tagtail, tail, testate, testator, testatrix,
   trainbearer, upholder, vindicator, votary, vouchsafer,
   walking stick, ward heeler, well-wisher