Despite a half-century of decline in membership and political influence, unions still play a prominent role in lobbying for congressional policy. For much of post-war history, policies around labor and employment have divided along partisan lines, seemingly confining labor’s influence to politicians in the Democratic Party. In this paper, we construct a dataset of labor and employment legislation from 1970 to 2020 to examine the influence of state-level union density on Republican Senators’ votes over the past half-century. We use a two-stage ordinary least squares model with a novel instrument to provide causal estimates of the effect of unionization on Senator voting behavior. The results suggest that the percentage of employed workers who are in unions in a state is strongly predictive of whether Republicans will defect in favor of labor policies supported primarily by Democrats.