This article explores how unions can influence employer decisions to downsize – a longstanding question that has been addressed through three waves of research. Although the literature has successively identified three types of factors that influence managerial decisions to downsize, it has not fully addressed the interactions of these factors, leading to inconsistencies. This article builds on and refines the existing work through a configurational approach which qualitatively evidences six downsizing configurations through a uniquely large number of cases ( n = 19). This research sheds new light on how factors combine to enhance union influence by theorizing four types of chemical-like factor interactions: catalysing, inhibiting, synthesizing and decomposing. By highlighting this interactive chemistry, this configurational approach revisits longstanding academic debates about the context–union strategy fit and the relative efficacy of union responses. From a practical perspective, it encourages unions to reject the idea that there is a single, optimal strategy for addressing downsizing.