AbstractIf any group of American blue-collar workers has benefited from the growth of trade it is the unionized dockworkers along the US West Coast. Nevertheless, the International Longshore and Warehouse Union (ILWU) representing these workers is vocally opposed to trade liberalization. We examine several competing explanations for this puzzle and evaluate them by tracing the union's stance on trade over several decades. We also use an original survey to compare ILWU affiliates' attitudes on trade with those of nonmembers with otherwise similar characteristics. Consistent with a model of organizational socialization, the data support the hypothesis that ILWU membership affects the members' revealed political opinions; the data are difficult to reconcile with standard theories of international trade. Our findings indicate that the political support for trade depends not just on voters' structural positions in the economy but also on the organizations and networks in which they are embedded.