When deciding whether to support a policy, citizens often rely on heuristics such as party cues, whether their party supports or opposes a policy proposal. While some argue this simplifies decision-making, critics warn it may mislead citizens when parties deviate from their typical policies. This study examines how the party system mitigates this risk. It proposes that cues from other parties serve as "fire alarms" for partisans, alerting them to deviations from their party's typical policies. Through 24 preregistered experiments involving partisans from all parties in the Danish parliament across three central issues, the study finds that the structure of party conflict shapes partisans' understanding and support for a policy from their party. Importantly, when policy proposals from partisans' own parties are opposed by parties with similar policy reputations to their own party and supported by parties with dissimilar policy reputations, partisans infer that the policy is atypical for their party and decrease their support. This shows how political institutions influence citizens' decision-making processes.