The Collaborative International Dictionary of English v.0.48:

Demagogue \Dem"a*gogue\ (?; 115), n. [Gr. dhmagwgo`s a popular
   leader; commonly in a bad sense, a leader of the mob; dh^mos
   the people + 'agwgo`s leading, fr. 'a`gein to lead; akin to
   E. act: cf. F. d['e]magogue.]
   A leader of the rabble; one who attempts to control the
   multitude by specious or deceitful arts; an unprincipled and
   factious mob orator or political leader.
   [1913 Webster]

WordNet (r) 3.0 (2006):

demagogue
    n 1: a political leader who seeks support by appealing to
         popular passions and prejudices [syn: demagogue,
         demagog, rabble-rouser]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "demagogue":
   agent provocateur, agitator, agitprop, catalyst, debate, debater,
   declaim, declaimer, elocute, exciter, firebrand, fomenter,
   harangue, haranguer, hold forth, hothead, incendiary, inciter,
   inflamer, instigator, jawsmith, mischief-maker, mouth, orate,
   out-herod Herod, panelist, perorate, provocateur, provoker,
   public speaker, rabble-rouse, rabble-rouser, rant, ranter, read,
   recite, ringleader, rodomontade, rouser, seditionary, seditionist,
   speaker, speecher, speechifier, speechmaker, spiel, spieler,
   spokesman, spokeswoman, spout, spouter, talker, troublemaker,
   tub-thump, tub-thumper, urger