This thesis analyses a new trade union model for umbrella company workers and the changes in the field of trade unions in Sweden. By conceptualizing trade unions as collective actors within strategic action fields I describe and explain how a small independent trade union and challenger in the field, Säljarnas Riksförbund, has managed to construct a new trade union model for umbrella company workers that differs from the traditional model used by the incumbent Unionen, the largest trade union in Sweden. Säljarnas Riksförbund challenges what a trade union club is and Unionen’s position in the field. Through two iterations of interviews with key representatives and document analysis of central documents, this case study provides empirical findings of how the new trade union model differs from the traditional model and how it poses a challenge for Unionen. There are three main findings, 1) The new trade union model has a divided organizational structure and works both as a part of the employer organization and the trade union and acts for both organizations, 2) The new trade union model operates primarily toward the state, unemployment insurance funds, and umbrella company workers. 3) The new trade union model organizes its members based on the constructed form of work, umbrella company work which challenges the traditional principle of organizing based on industrial or occupational belonging. The study finds that the incumbent and challenger did not perceive the stakes in the field similarly, the internal governance units have not hindered the emerging challenge, and a change in a related field has not only destabilized the field in question but also invaded it. An episode of contention is emerging and the rules in the field are in flux., This study explores how trade unions organize umbrella company workers in Sweden. It focuses on a new trade union club that has been created by Säljarnas Riksförbund together with the umbrella company Frilans Finans. These two labor market parties established the first collective agreement and the first trade union club for umbrella company workers in Sweden. This is studied in relation to Unionen, the largest trade union in Sweden, which previously decided not to conclude a collective with the same umbrella company. The question is how and why did Säljarnas Riksförbund chose to conclude a collective agreement whereas Unionen did not? Säljarnas Riksförbund has taken the initiative from Unionen to become the trade union for umbrella company workers. Against this background, I pose three questions with the aim of understanding and exploring how this new trade union club is organized, how it works, and how it changes the field of trade unions. By designing this thesis as a case study and conducting two rounds of interviews with key representatives of each organization and analyzing the central documents of these organizations this thesis provides answers. The analysis shows that the new trade union club in fact is a new trade union model, a new way of organizing members, and a new way of acting as a trade union. This new model is an alternative model to the traditional one and thus challenges what a trade union is and what it does. The analysis shows that Säljarnas Riksförbund, through their alternative union model poses a challenge to the traditional trade union model of organizing employees and to Unionen’s position in the field. The new union club is both a member of the employer’s company and the trade union. The new union club maintains a dialogue with the employer through processes within the company itself and works for both organizations toward the state, unemployment insurance funds, and umbrella company workers. The new trade union model organizes umbrella co