The Collaborative International Dictionary of English v.0.48:

Objector \Ob*ject"or\, n. [L., an accuser.]
   One who objects; one who offers objections to a proposition
   or measure.
   [1913 Webster]

WordNet (r) 3.0 (2006):

objector
    n 1: a person who dissents from some established policy [syn:
         dissenter, dissident, protester, objector,
         contestant]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "objector":
   apostate, bitter-ender, brawler, defendant, demurrer, diehard,
   disputant, dissenter, dissentient, dissident, intransigent,
   irreconcilable, last-ditcher, litigant, naysayer, negativist,
   nonconformist, noncooperator, obstructionist, obstructive,
   opinionist, opposer, opposition voice, oppositionist, plaintiff,
   protestant, protester, quarreler, recusant, resister, schismatic,
   scrapper, sectarian, sectary, separatist, wrangler