(n.) One who objects; one who offers objections to a proposition or measure\.

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