The Collaborative International Dictionary of English v.0.48:

Seconder \Sec"ond*er\, n.
   One who seconds[3] or supports what another attempts,
   affirms, moves, or proposes; as, the seconder of an
   enterprise or of a motion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seconder
    n 1: someone who endorses a motion or petition as a necessary
         preliminary to a discussion or vote

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "seconder":
   Maecenas, abettor, admirer, advocate, aficionado, angel, apologist,
   backer, buff, champion, defender, dependence, encourager, endorser,
   exponent, fan, favorer, friend at court, lover, mainstay,
   maintainer, paranymph, partisan, patron, promoter, protagonist,
   reliance, second, sectary, sider, sponsor, stalwart, standby,
   support, supporter, sustainer, sympathizer, upholder, votary,
   well-wisher