Why, despite experiencing similar systemic shocks, has Bolivia embarked on a successful process of (partial) party system reconstruction while Peru’s party system remains collapsed? This dissertation introduces a theory of networked cleavage articulation that explains this variation in party system reconstruction outcomes. This theory posits that the social network structures within which nascent parties emerge condition the articulation of social cleavages in the party system and define the extent to which cleavages become associated with party system consolidation or increased political instability. The dissertation employs original interviews, surveys, and electoral data to evaluate this theory through a mixed-methods approach. It offers three central findings. First, it demonstrates that social cleavages can consistently structure political behavior and nonetheless be associated with various party system outcomes, from consolidation to social conflict. I show that, ethnic cleavages in Bolivia and Peru have structured political behavior since their democratic transitions and throughout major party system transformations. Second, the dissertation demonstrates that the degree of cleavage articulation explains variation in party system reconstruction outcomes. In Bolivia, the MAS-IPSP produced a successful articulation of the indigenous bloc of the ethnic cleavage, enabling partial party system reconstruction. In contrast, political parties’ articulation offers for the non-indigenous in Bolivia and both the indigenous and non-indigenous in Peru have thus far failed. Consequently, these ethnic blocs remain associated with political instability. Finally, this dissertation shows that the likelihood of cleavage articulation is conditioned by nascent parties’ original network landscapes. I find that, in contrast to other indigenous and non-indigenous parties in Bolivia and Peru, which have emerged from networks with regional strength and limited paths to expansion, the MAS-IPSP emerged within a vast organizational landscape that provided it with diverse within-network paths to growth and enabled it to produce a successful indigenous bloc articulation. This dissertation challenges current understanding of the relationship between social cleavages and party systems. By bringing attention to the challenges of cleavage articulation, its likelihood of failure, and the role of social networks in this process, the dissertation introduces new and important insights to existing literature on social cleavages, political representation, and party politics.