The Collaborative International Dictionary of English v.0.48:

Proponent \Pro*po"nent\, a. [L. proponens, p. pr.]
   Making proposals; proposing.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Proponent \Pro*po"nent\, n.
   [1913 Webster]
   1. One who makes a proposal, or lays down a proposition.
      --Dryden.
      [1913 Webster]

   2. (Law) The propounder of a thing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

proponent
    n 1: a person who pleads for a cause or propounds an idea [syn:
         advocate, advocator, proponent, exponent]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "proponent":
   adherent, advocate, apologete, apologist, apologizer, backer,
   champion, defender, enthusiast, exponent, expounder, friend,
   justifier, partisan, patron, pleader, promoter, proposer,
   spokesman, spokesperson, spokeswoman, subscriber,
   successful advocate, supporter, upholder, vindicator,
   whitewasher



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PROPONENT, eccl. law. One who propounds a telling as "the party proponent
doth allege and propound." 6 Eng. Ecclesiastical R. 356, n.