Extant research in political science has demonstrated that citizens’ opinions on policies are influenced by their attachment to the party sponsoring them. At the same time, little evidence exists illuminating the psychological processes through which such party cues are filtered. From the psychological literature on source cues, we derive two possible hypotheses: (1) party cues activate heuristic processing aimed at minimizing the processing effort during opinion formation, and (2) party cues activate group motivational processes that compel citizens to support the position of their party. As part of the latter processes, the presence of party cues would make individuals engage in effortful motivated reasoning to produce arguments for the correctness of their party’s position. Following psychological research, we use response latency to measure processing effort and, in support of the motivated reasoning hypothesis, demonstrate that across student and nationally representative samples, the presence of party cues increases processing effort.