The idea that U.S. conservatives are uniquely likely to hold misperceptions is widespread but has not been systematically assessed. Research has focused on beliefs about narrow sets of claims never intended to capture the richness of the political information environment. Furthermore, factors contributing to this performance gap remain unclear. We generated an unique longitudinal dataset combining social media engagement data and a 12-wave panel study of Americans’ political knowledge about high-profile news over 6 months. Results confirm that conservatives have lower sensitivity than liberals, performing worse at distinguishing truths and falsehoods. This is partially explained by the fact that the most widely shared falsehoods tend to promote conservative positions, while corresponding truths typically favor liberals. The problem is exacerbated by liberals’ tendency to experience bigger improvements in sensitivity than conservatives as the proportion of partisan news increases. These results underscore the importance of reducing the supply of right-leaning misinformation.