“A demagogue, in the strict signification of the word, is a 'leader of the rabble'.”                         — James Fenimore Cooper, "On Demagogues"

a political leader who seeks support
by appealing to popular desires &
prejudices rather than by using rational argument;
A demagogue or rabble-rouser is a leader
in a democracy who gains popularity
by exploiting prejudice & ignorance
among the common people, whipping up the passions
of the crowd & shutting down reasoned deliberations;
rabble-rouser, agitator, political agitator,
soapbox orator, firebrand, fomenter, provocateur
"he was drawn into a circle of campus demagogues"
Only in ancient Greece and Rome
was it a leader or orator who espoused
the cause of the common people;
demagogues overturn established customs of political conduct,
or promise or threaten to do so;
demagogues have appeared in democracies
since ancient Athens. They exploit a fundamental
weakness in democracy: because ultimate power
is held by the people, it is possible for the people
to give that power to someone who appeals
  to the lowest common denominator
of a large segment of the population;
demagogues usually advocate immediate,
forceful action to address a national crisis
while accusing moderate & thoughtful opponents
                                       of weakness or disloyalty