How do party representatives as poll workers at electoral tables (voting booths) affect elections? Several countries across the globe have a system of counting votes that is partisan and adversarial, where poll workers are party representatives. In theory this system ensures the integrity of elections by legally requiring that multiple parties count votes at each booth. Yet in countries with dominant party regimes and weak state capacity, parties often have de facto unequal capacities to send representatives to all booths. We show this is the case during the 2018 Paraguayan elections we study, leading to stark inequalities in partisan poll worker representation. We estimate that partisan poll workers decrease an opposing party's vote share by up to 2 percentage points (pp) and increase theirs by up to 1 pp. Our analyses also demonstrate how incentives for electoral manipulation vary by electoral system. We find that partisan poll workers collude more often with rival party poll workers within proportional representation races, as distributing votes among themselves can help their parties earn more seats for their legislative candidates. In contrast, single-winner plurality voting yields less collusion, as partisan poll workers compete against all parties running against their own. Our results have practical implications for political parties and policymakers, as well as theoretical implications for electoral competition in developing democracies.