Why do some new political parties take root after rising to electoral prominence, while others collapse after their initial success? Although strong parties are critical for stable, high-quality democracy, relatively little is known about the conditions under which strong parties emerge. The classic literature on party-system development is largely based on studies of the United States and Western European countries. 1 Since almost all of these polities developed stable party systems, the classic theories tend to take successful party-building for granted. While telling us much about how electoral rules, social cleavages, and access to patronage shape emerging party systems, they leave aside a more fundamental question: Under what conditions do strong parties emerge in the first place? Since the onset of the third wave of democratization, attempts to build parties have failed in much of the developing world. 2 In Latin America, over 95 percent of the parties born during the 1980s and 1990s disbanded after failing to take off electorally, and even among the small subset of parties that attained national prominence, most collapsed shortly afterward. 3 Despite the preponderance of unsuccessful new parties, existing literature on party-building in developing countries focuses overwhelmingly on the tiny fraction of new parties that survived. For example, scholars have written hundreds of book-length studies on successful new parties in Latin America but only a few such studies on unsuccessful cases. 4 This inattention to unsuccessful cases is methodologically problematic: without studying cases of party-building failure, we cannot fully account for party-building success. Latin America's "new left" parties-left parties born during the region's third wave (1978-1995)-provide a rich universe of cases for analyzing variation in partybuilding outcomes. 5 These parties faced the same initial challenges. With the collapse of ISI and the emergence of an elite neoliberal consensus in the 1980s and 1990s, the left's traditional economic platform became politically infeasible in much of the region, and with the decline of labor unions and rise of the informal sector, the left's capacity to mobilize the popular classes decreased considerably. 6 Despite these common challenges, Latin America's new left parties experienced widely varying degrees of success. The overwhelming majority flopped electorally and quickly disappeared, 7 while roughly a dozen attained national electoral prominence. Of these roughly dozen, five took root and are now institutionalized parties, while the others collapsed after their initial success. 8 The survival and collapse of Latin America's new left parties had major consequences. Where new left parties survived, party systems and democratic regimes were more likely to become consolidated (e.g., Brazil). 9 Where new left parties collapsed, checks on executive power and opposition to dominant parties weakened (e.g., Argentina), and party systems and democratic regimes were more likely to break down (e.g.,...