Many successful political parties depend for their initial popularity and cohesion, and even for their long-term brand strength, on a leader. Nevertheless, literature on successful party building downplays the role of leaders. Thus, the question, what type of leader is good for party building?, remains undertheorized. This article presents and provides initial evidence for a leadership-centered theory of successful party building. It argues that externally appealing, internally dominant leaders facilitate party building by lifting new parties to electoral prominence and helping to prevent debilitating schisms. The article provides evidence for this argument through a most similar cases comparison of three new left parties in Latin America: two that took root (Brazil’s Workers’ Party, Mexico’s Party of the Democratic Revolution), and one that collapsed (Peru’s United Left).