Creating new parties is hard. It requires the development of permanent or at least semipermanent mechanisms of (1) horizontal coordination and (2) vertical interest aggregation. In Latin America, new “electoral vehicles” – organizations that do not meet one or more of the above criteria – are everywhere. Few of these electoral vehicles become political parties in the full sense of the term This chapter offers an account of one of Latin America’s rare party-building successes – the Bolivian MAS (Movement toward Socialism). It explains an under-theorized path to party-building, via autonomous social movements, and shows how movements can shape party organizational models. The analysis reveals that the MAS meets the two criteria for successful party building but retains strikingly fluid organizational attributes. In the absence of strong national and local party structures that can serve as “transmission belts,” it accomplishes horizontal coordination and vertical aggregation through predominantly informal channels, rather than through party structures. The chapter describes these informal channels and discusses their effects on broader issues related to democratic representation, responsiveness, and accountability.