This paper analyzes Jan-Werner Müller’s and Chantal Mouffe’s influential positions on populism as a strategy for the Left in order to clarify what is at stake in current debates about populism and, thus, to identify the political challenges more clearly. Müller and Mouffe both start from different versions of post-foundationalism, but understand post-foundationalism differently, and this has implications for how they understand democracy, populism and the legitimacy of the constitutional order. The different understandings of post-foundationalism also have implications for the question of how to guarantee pluralism, which is so central to debates about populism today. Pushing Müller’s post-foundationalism to its logical conclusions, I show how he cannot uphold the distinction he makes between democracy and populism. Mouffe goes further in her post-foundationalism, but she too tries to control the effects of the post-foundationalist starting point in ways that are ultimately untenable.