This article states that, since the return to democracy in Chile, the Central Unitaria de Trabajadores (CUT) has played a double role for unionism. On the one hand, it has sought to promote, with very few resources and little success, new legislation for the private sector and, on the other, it has defended, with many resources and much success, the autonomy and interests of the public sector. How has this dual role played by the CUT since the return to democracy impacted on the identities and organizational practices of Chilean unions? To answer this question,it is proposed the concept of union cleavages, which describes a division that is simultaneously structural, ideological and organizational within Chilean unionism. Based on a historical analysis, it is described how the actions of the CUT were politicizing the structural division between the public and private sectors of unionism, consolidating collective identities around that division. By means of a quantitative analysis of the alliance networks in labor strikes from 2010 to 2019, the differentiated behavior of the main federations is described, according to whether they belong to the public or private sector. The results show that the public sector federations are the ones that find the highest density of alliances between them, generating a cleavage that is clearly differentiated from the private sector unionism, which is socially and politically isolated. The importance of considering this fracture within Chilean trade unionism is highlighted as a highly explanatory element of the limitations and strengths of CUT's political capacity.