This paper argues that parties are the most critical actors developing, impeding, or undermining democratic institutions in the post-Arab spring Middle East cases of Egypt, Iraq, Libya, and Tunisia. We argue that party behavior in these nascent democracies is deeply conditioned by the development of cleavage structures in the prior authoritarian period. We trace the development of cleavage structures, explaining the absence of left-right cleavages and the prevalence of either universalist-transformative or particularistic-redistributive cleavage structures. We then modify classic models of democratization, recognizing that contenders weigh costs of toleration and suppression when they decide whether to accept or subvert democratic institutions. We go beyond the conventional emphasis on economic distributional issues, however, to understand how the cleavages mobilized by political parties shape these relative costs and hence set the parameters for party behavior and the likelihood of democratic consolidation.