The landslide victory of the center-left Moon Jae-In in the 2017 presidential election opened a window of opportunity for progressive reform in South Korea. Elected on a platform of social inclusion and fairness and with support from organized labor, the Moon Jae-In government (2017–22) made at first considerable progress in advancing inclusive labor market reform and an alternative growth model against opposition from business, but the administration quickly lost momentum when facing political headwinds. We show that the government's capacity for progressive reform and social concertation was constrained not only by business interests but also by divisions on the left. Opening up the black box of organized labor, we provide a nuanced analysis of tensions on the left and demonstrate how a counterintuitive coalition of labor market insiders and radical outsiders on the movement's left undermined social dialogue and more inclusive unionism.