A leader who uses a number of underhanded political techniques in order to gain power, including: warmongering, fear, censorship, propaganda, nationalism, xenophobia, hateful rhetoric, false logic, and blatant lies. Uses the blind, patriotic support of the people to institute a corrupt and oppressive government.
Some famous historical demagogues include Hitler, Stalin, Mussolini, and Bush, Jr.
A leader who obtains power by means of impassioned appeals to the emotions and prejudices of the populace.
A leader of the common people in ancient times.
Note that in modern usage the word is derogatory (negative).
Bill Clinton - The ultimate demagogue. Scandals of the Clinton Administration - sex, murder, illegal campaign funds, white water.