To what extent can electoral engineering mitigate deadly intra-state conflict? This paper investigates the impact of electoral engineering on nationalist party behavior in highly-fragmented states. As nationalist parties have been instrumental in escalating inter-group tensions to large-scale hostilities, frameworks for conflict resolution frequently incorporate institutional mechanisms as a means of altering the incentives for conflict exploitation or for inter-group cooperation. Specifically, the paper investigates proportional representation (PR) and preferential systems. To test the impact of these systems, the study observes party engagement in cooperative or conflictual behavior during legislative campaigns in the Federation of Bosnia-Herzegovina, Republika Srpska, and Croatia over several election cycles. Data from the Bosnian cases largely support expectations that PR presents incentives for nationalist parties to “play the ethnic card” and exacerbate communal conflict. In contrast, the Croatian case provides a degree of support for the prediction that preferential voting structures encourage cooperation and thus conflict dampening.