(n.) One who belongs to the opposition party.

agitator battler belligerent belted knight bickerer bitter-ender blade bravo bully bullyboy combatant competitor contender contestant defendant diehard disputant dissentient dissident duelist enforcer extremist fencer feuder fighter fighting cock foilsman frondeur gamecock gladiator goon gorilla hatchet man hood hoodlum hooligan insubordinate insurgent insurrectionary insurrectionist insurrecto intransigent irreconcilable jouster knight last-ditcher litigant malcontent maverick militant mutineer naysayer negativist nonconformist noncooperator objector obstructionist obstructive opposer oppositionist plaintiff plug-ugly protester quarreler rebel resister revolter revolutionary revolutionist rioter rival rough rowdy ruffian sabreur scrapper scuffler squabbler strong arm strong-arm man strong-armer struggler subversive swashbuckler sword swordplayer swordsman thug tilter tough traitor tussler wrangler