The development of methods to measure the ideological positions of legislators and other political actors has become one of the most successful research programs in the study of political behavior. Less effort, however, has been given to approaches for measuring public uncertainty about those positions. In this article, we propose the use of a well-known measurement model to investigate public uncertainty about the ideological locations of political actors. To motivate its use empirically, we examine citizens’ perceptions of the ideology of Donald Trump during the 2016 election campaign. We show that among Republican and Democratic candidates since 1972, and compared to contemporary legislators, citizens have had the most difficulty assessing the ideology of Donald Trump. Our approach is widely applicable to questions concerning public perceptions of politicians’ ideology and the political behavior of citizens and their representatives.