This dissertation explores how religious teachings about economic injustice and the moral obligation to address the economic and political roots of poverty shape religion's influence on political competition. At least since Marx, religion has been thought to decrease electoral support for left parties and undercut redistribution. Yet following Weber, scholars have also documented religious denominations' distinct perspectives about the economic sphere and the ways in which these views shape individuals' beliefs and behavior. I examine the effect of religious teachings on the electoral fortunes of left-wing political parties, specifically. While some religious denominations encourage believers to embrace affluence and view poverty as a consequence of individual effort (or lack thereof), others emphasize poverty's injustice and advocate structural changes to alleviate this economic hardship. I focus on the political influence of the latter---which I refer to as progressive religion---and study its effects on the organizational development, electoral support, and policy priorities of left-wing political parties.

I argue that progressive religion can provide critical organizational and electoral resources to left-wing political parties. When progressive religious leaders underscore the injustice of poverty and foster support for economic redistribution among their parishioners, they create a group of religious voters whose economic preferences align with those of left-wing political parties. Moreover, the emphasis on addressing poverty through structural reforms in society encourages parishioners to actively engage in social and political movements that prioritize these goals. In addition to their predisposition to support the economic policies that left-wing political parties champion, pious voters are thus also likely to become active and organized members of the left. Their support allows left parties to complement and extend their territorial presence by relying on an array of lay organizations. Hence, progressive religion provides the left with important resources in the electoral arena.

Yet the support of progressive religion does not come without consequences for left parties' policy platform. While pious voters share the left's preferences about economic redistribution and other reforms to reduce economic inequality, they are conservative with respect to social issues such as abortion and gay marriage. When these voters support left parties, they bring their socially conservative preferences to bear on the parties' policy agenda and oppose any effort to embrace non-conservative positions on these social issues. As a result, left parties that cultivate the support of progressive religion will adopt socially conservative positions.

I support these claims using a multi-method research strategy that combines qualitative, quantitative, and quasi-experimental evidence from the Catholic Church in Brazil. I first combine a historical analysis of Catholic social doctrine with a study of Brazilian diocesan publications to document variation in Catholic teachings on economic issues, particularly with respect to the roots of poverty and the need to address the economic inequality it engenders in the political realm. I then test my argument using original archival data and drawing on a natural experiment in Brazil following the transition to the papacy of Pope John Paul II in 1978. Leveraging plausibly as-if random variation in bishop vacancies that led some Brazilian dioceses to be overseen by progressive bishops while exposing others to conservative bishops appointed by Pope John Paul II, I study the effect of progressive bishops on religious involvement in progressive economic causes as well as the electoral success of the left-wing Workers' Party (PT). I find that the Catholic Church was significantly more likely to support progressive economic causes in dioceses led by economically progressive bishops and that the left enjoyed a remarkable electoral advantage in these areas. The party's stronger performance can be partly explained by its access to religious networks that allowed it to develop an organizational structure, particularly in areas where the party lacked an existing base of electoral support. However, the electoral benefits of progressive religion came at the cost of marginalizing the political demands of the party's feminist faction, thereby decreasing the PT's adoption of gender policies at odds with religious teachings.

Substantively, these findings contribute to the growing body of evidence that religious teachings shape religion's effect on political behavior. They also suggest that religion can serve as a critical source of support for economically progressive political and social movements. Moreover, this study contributes to our understanding of the broader policy considerations that left-wing parties face. My analysis suggests that socially progressive policy positions are not universally constitutive of left parties. Instead, left parties' reliance on partners to build their national infrastructure can lead them to strategically select a social policy position in order to capitalize on the organizational reach of existing social institutions. The recent decline in rates of industrialization and weakening of labor unions across both the developing and developed world has forced the left to seek new sources of organizational support in order to maintain its political relevance. The mobilizational capacity of religious organizations, particularly in the developing world, makes them an attractive partner for left parties to accomplish this goal.