Theories of low-information rationality claim that uninformed voters can compensate for their lack of political knowledge by employing heuristics, such as interest group endorsements, to make voting decisions as if they were fully informed. Critics of low-information rationality contend that politically unaware voters are unlikely to use group endorsements effectively as a heuristic since they are unlikely to know the political relevance of interest groups. We address this debate by entertaining the possibility that contextual information coupled with a source cue may enhance the effectiveness of group endorsements as a heuristic. We test competing expectations with a field experiment conducted during the 2006 election in two highly competitive Pennsylvania statehouse races where a well-known liberal interest group endorsed Democratic candidates and canvassed both core supporters and Republicans believed to be likeminded. Our results reveal that Republicans used the endorsement as a negative voting cue and that the group's endorsement helped some Republicans compensate for their lack of awareness about politics.