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]