This article examines the American labor movement's struggles since the nineteenth century over how to respond to mass immigration. Labor's struggles have turned on whether it views new waves fundamentally as a threat, which elicits a strategy of restriction, or an opportunity, which elicits a strategy of solidarity. It also captures the advantages of a longue-duree approach for understanding the fraught and evolving relationship between American unionism and immigration. Rather than a Briggsian story of labor traditionally embracing a restrictionist position, our archival and interview research from the Reconstruction Era to present shows that labor's position on immigration has been in regular contention—with disagreements getting resolved in a restrictionist direction during certain periods and an expansionist one during others. Likewise, the familiar scholarly claim that an unprecedented “turnabout” in labor's response to immigration can be pinpointed to 1999 ignores more than a century of internal debate and variegated external activism on this issue. We lay out an analytical model for understanding why the labor movement has viewed new immigrant workers as a threat in certain contexts and an opportunity for growth in others. The model highlights how three external variables—the fluid structure of the labor market, immigration trends, and the state's disposition toward organized labor—establish either a secure or insecure environment within which unions respond to immigration. It also underscores the importance of how dominant modes of unionism within the movement interact with these external forces to shape its perception of “new” immigrants in restrictive or solidaristic terms. Significantly, the sequence and recombination of these forces during the past century or more have transformed how organized labor responds to new immigrant workers in an insecure environment today. Our research presents a diverse movement honestly wrestling with immigration's profound conundrums, including elemental issues of who it identifies as part of its fold (workers deserving of fraternity and sorority) and who it deems as permanent outsiders (workers who were a menace to the cause).