The Collaborative International Dictionary of English v.0.48:

diehard \die"hard`\ (d[imac]"h[aum]rd`), n.
   1. one who stubbornly adheres to traditional and outdated
      views.

   Syn: traditionalist.
        [WordNet 1.5]

   2. one who vigorously defends an apparently hopeless
      position, a lost cause, etc.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

diehard \die"hard`\ (d[imac]"h[aum]rd`), a.
   stubbornly and vigorously resisting in the face of seemingly
   hopeless odds; as, diehard opposition.

   Syn: never-say-die. [PJC]

WordNet (r) 3.0 (2006):

die-hard
    adj 1: tradition-bound and obstinately opinionated; "an
           inflexible (or die-hard) conservative"; "rock-ribbed
           republican" [syn: die-hard(a), rock-ribbed]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "diehard":
   Bircher, Bourbon, Tory, ass, bigot, bitter-ender, brawler,
   bullethead, conservationist, conservatist, conservative, defendant,
   disputant, dissentient, dissident, dogmatist, donkey,
   extreme right-winger, fanatic, fogy, fundamentalist, hard hat,
   hardnose, imperialist, intransigeant, intransigent, irreconcilable,
   last-ditcher, laudator temporis acti, litigant, maverick,
   monarchist, mossback, mule, naysayer, negativist, noncooperator,
   objector, obstructionist, obstructive, old, old fogy, old school,
   opposer, oppositionist, perverse fool, pighead, plaintiff,
   positivist, protester, purist, quarreler, radical right,
   reactionarist, reactionary, reactionist, resister, right,
   right wing, right-winger, rightist, royalist, scrapper,
   social Darwinist, standpat, standpatter, stick-in-the-mud,
   stickler, ultraconservative, white, wrangler