The extent to which political conflict over U.S. trade policy has led to clashes between broad-based class coalitions has varied significantly over time during the past two centuries. I argue that much of this variation can be explained by changes in economywide levels of interindustry factor mobility. Class distinctions between voters are more economically and politically salient when interindustry mobility is high; when mobility is low, industry distinctions become more critical and tend to split apart broader political coalitions. I report evidence indicating large changes in levels of labor and capital mobility over the last two centuries. These changes coincide with significant shifts in the character of American trade politics. Analysis of congressional voting on 30 major pieces of trade legislation between 1824 and 1994 provides evidence of large swings in coalition patterns.