This paper analyzes how democratization in Southeast Asia has led to the politicization of social and economic inequalities since the 1990s. Drawing on political attitudes surveys, we document how historical legacies, the structure of inequality, and institutional dynamics contributed to differentially structure political cleavages in Indonesia, Malaysia, Thailand, and the Philippines. Ethnic inequalities in Malaysia and regional inequalities in Thailand played a key role in fostering the emergence of strong class divides in these two countries. In Indonesia, by contrast, the rise of new parties mainly used as political vehicles to support their leaders has eroded pre-existing religious and socioeconomic cleavages. The Philippines' highly unstable party system led to a third trajectory, whereby class divides have been strong, but have lacked the conditions required for their institutionalization and stabilization over time.