Which firms support trade liberalization and under what circumstances? The dominant approaches to trade politics ignore two key features of modern international commerce -- firm heterogeneity in export performance and intra-industry trade -- which jointly imply that industries will be divided over bilateral trade liberalization. This dissertation examines the impact of these features on the politics of trade, exploring the preferences of firms, the attitudes of industries, and the motivations of politicians, in turn. When products are differentiated, firms which do not export generally oppose trade liberalization even in industries at a comparative advantage relative to their foreign trade partners. Not all exporting firms will be supporters of trade, however. For example, the largest exporters may oppose trade liberalization in their export markets due to increased competition from compatriot firms. It is then argued that industries are most likely to be divided where product differentiation is high and differences in competitiveness between trade partners are muted. This pattern is documented empirically in a study of US industries' attitudes toward the US-Korea and US-Australia Free Trade Agreements. Finally, a complete political economic model of trade policy determination with heterogeneous firms is developed. The changing preferences of politicians across different economic and institutional settings are explored, and comparative statics identified which show how equilibrium tariffs change with key industry features.
