Political polarization threatens democracy in America. This article helps illuminate what drives it, as well as what factors account for its asymmetric nature. In particular, we focus on positive feedback among members of Congress as the key mechanism of polarization. We show how public opinion, which responds to the laws legislators make, in turn drives the feedback dynamics of political elites. Specifically, we find that voters’ “policy mood,” i.e., whether public opinion leans in a more liberal or conservative direction, drives asymmetries in elite polarization over time. Our model also demonstrates that once self-reinforcing processes among elites reach a critical threshold, polarization rapidly accelerates. By tying together elite and voter dynamics, this paper presents a unified theory of political polarization.