This paper compares two influential but conflicting contemporary models of politics as an activity: those of Hannah Arendt and Alain Badiou. It discovers the fundamental difference between their approaches to politics in their opposing evaluations of the contemporary political significance of the legacy of Plato, Platonism, and the Platonic Idea. Karl Popper’s and Arendt’s analyses of the inherently ideological nature of totalitarianism are contrasted with Badiou’s vindication of an ideological “politics of the Idea.” Arendt and Badiou are shown to share an understanding of politics as a realm for the human deployment of novelty and world-transformation. Their key disagreement concerns the form of activity that accomplishes this deployment. For Arendt, political activity has the basic form of noninstrumental and nonteleological action (praxis), devalued by the Platonic tradition of political philosophy. Badiou, by contrast, follows Plato in regarding politics essentially as a process of production (poiēsis) oriented to an ideal end.