Past the half-century mark of Converse's (1964) field-defining essay, the nature of political ideology in the mass public and how it has changed in response to partisan polarization remains enigmatic. To test the ideological structure of US public opinion, I develop and implement a Bayesian dynamic ordinal item response theory model. In contrast to static scaling procedures, this method allows for changes in the mappings between issue attitudes and the underlying ideological dimension over time. The results indicate that over the last forty years, mass attitudes on a range of long-standing policy controversies better fit a unidimensional ideological structure. As among elites, the left–right dimension has come to encompass a wide range of policy, partisan, and value divides in the mass public. Further, these trends hold for voters at all levels of political sophistication. Widespread conflict extension appears to be a defining feature of mass polarization in contemporary US politics.
