This article explores the coalitional success of mass-mobilizing, reformist parties once they achieve power. Why are some of these parties more successful than others at managing the potentially conflicting interests of their diverse social bases? We argue that organizational strategies adopted early on matter greatly. The nature of the party’s core constituency, together with the linkage strategies undertaken by party leaders in crafting a coalition of support, shapes a party’s ability to maintain that coalition over time. When coalitional partners are intensively rather than extensively integrated, they are more likely to compromise over policy disagreements rather than defect when defection becomes attractive. We develop this theory by comparing the evolution of two Bolivian parties: the Revolutionary Nationalist Movement and the Movement Toward Socialism. Against conventional explanations that are overly dependent upon structural factors, our argument stresses the impact of strategic choices in shaping a party’s ability to maintain its coalition.