A demagogue (from Greek δημαγωγός, a popular leader, a leader of a mob, from δῆμος, people, populace, the commons + ἀγωγός leading, leader) or rabble-rouser is a political leader in a democracy who gains popularity by arousing the common people against elites, especially through oratory that whips up the passions of crowds, appealing to emotion by scapegoating out-groups, exaggerating dangers to stoke fears, lying for emotional effect, or other rhetoric that tends to drown out reasoned deliberation and encourage fanatical popularity. Demagogues overturn established norms of political conduct, or promise or threaten to do so.^: 32–38

Historian Reinhard Luthin defined demagogue as "...a politician skilled in oratory, flattery and invective; evasive in discussing vital issues; promising everything to everybody; appealing to the passions rather than the reason of the public; and arousing racial, religious, and class prejudices—a man whose lust for power without recourse to principle leads him to seek to become a master of the masses. He has for centuries practiced his profession of 'man of the people'. He is a product of a political tradition nearly as old as western civilization itself."^: 3

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.^: 31–71 Demagogues have usually advocated immediate, forceful action to address a crisis while accusing moderate and thoughtful opponents of weakness or disloyalty. Many demagogues elected to high executive office have unraveled constitutional limits on executive power and tried to convert their democracy into a dictatorship, sometimes successfully.