Political parties in new democracies are frequently regarded as prone to (extreme) organizational weakness and vulnerable to swift death. This dissertation examines the factors that facilitate parties' organization-building success in one subset of new democracies. It relies on a multi-method approach that combines quantitative data from ten Latin American countries and qualitative evidence from two Chilean parties to demonstrate that electoral promise helps parties' organizations develop and survive. Defined as an indication that a party is likely to obtain a proportion of the vote that would allow it to become or remain a significant actor on its national political scene, electoral promise positively affects parties' organizational development through two main mechanisms. First, it attracts (or helps retain) candidates' and activists' loyalty by ensuring that the party has access to resources it could use to provide its members with selective incentives, such as elected positions or patronage in the form of jobs. Second, it ensures that the party has elected officeholders. The latter are important because they are often capable of maintaining the party's structures even in the absence of a concerted leadership effort to do so. Indeed, officeholders themselves rarely set out to work for their organizations. Rather, they frequently end up keeping local branches alive and active by engaging in behaviors perceived as beneficial to their re-election.I show how these dynamics work by focusing on the development and subsequent maintenance of two strong partisan organizations in Chile – the Party of Christian Democracy and the Chilean Socialist Party. My account follows these parties from their respective inceptions, through the polarization of the 1960s and early 1970s, Pinochet's dictatorial regime, and five terms in government to emphasize the importance of electoral promise over the past 30 years.