Since the end of World War II, democracies typically fell apart by coup d'état or through force. Today, however, they are increasingly eroding at the hands of democratically elected incumbents, who seize control by slowly chipping away at democratic institutions. To better understand these developments, this book examines the role of personalist political parties, or parties that exist primarily to further their leader's career as opposed to promote a specific policy platform. Using original data capturing levels of personalism in the parties of democratically elected leaders from 1991 to 2020, The Origins of Elected Strongmen shows that the rise of personalist parties around the globe is facilitating the decline of democracy. Personalist parties lack both the incentive and capacity to push back against a leader's efforts to expand executive power. As such, leaders backed by personalist parties are more likely to succeed in their efforts to dismantle institutional constraints on their rule. Such attacks on state institutions, in turn, reverberate throughout society, deepening political polarization and weakening supporters' commitment to democratic norms of behaviour. In these ways, ruling party personalism erodes horizontal and vertical constraints on a leader, ultimately degrading democracy and raising the risk of democratic failure.