Why do new parties split? Scholars of new party schisms shy away from leadership-centred explanations for fear of excessive voluntarism and thus fail to conceptualise differences between leaders systematically. This article challenges that trend, arguing that externally appealing, internally dominant leaders generate cohesion in new parties. It analyses why some externally appealing leaders are internally dominant, while others are not, and argues that this variation can make the difference between schism and survival. The article supports its argument through a representative case study: the fatal (and consequential) schism of Peru's United Left coalition in the late 1980s.