Is the Left reemerging as a political force? If we are indeed seeing a new phase of American radicalism, it would not be the first time. Based on insights from the online Mapping American Social Movements Project, this essay reframes the history of American radicalism by paying close attention to the singular fact that for the last century the Left has consisted solely of shifting constellations of social movements without the anchoring presence of a competitive left-wing electoral party. As a result, the American Left has been uniquely unstable. Its organizations come and go, flourishing for a time then withering, only to be replaced at some later point by a new Left based in different organizations, often with different demography, geography, and ideological agendas. This article maps five distinct left constellations over the past century and explores the question of how American radicalism has repeatedly reconstituted itself absent the supportive institutional apparatus of an electoral party.