Compared to other Western democracies, in the United States fewer people subjectively identify as working class historically and many working-class individuals think of themselves as middle class. This likely has important political implications. We argue, however, that union membership can strengthen identification with the working class, through communications from leaders and interactions among members. Using General Social Survey data from five decades, we develop an original multi-indicator IRT-based measure of objective class status and find that union membership makes it more likely that individuals identify as working class, across all objective class groups. Panel data analysis shows that union membership predicts future working-class identification but that the opposite is not true, suggesting that these associations are causal. Finally, we show that identifying with the working rather than middle or upper class is associated with more support for redistribution and the welfare state.