For a trade union delegate, ‘campaigning on a daily basis’ takes place on multiple fronts. It is also at this operational level that the challenges of the militant ‘organisational work’ that this article specifically addresses. He reported on this on the basis of the results of a wide-ranging research and action carried out by around 100 activists from the largest Belgian trade union. After clarifying the methodological options of the survey and some general challenges for the grassroots trade union activism, the article successively develops the tensions at the three levels identified for the deployment of militant organisation work. In this context, he highlighted the impact of the strategic differences between the organisation and the field, the great challenge of developing a trade union culture in organisation/business and the difficulties relating to the integration of new recruits. The conclusion will include some valuable challenges to the work of a militant organisation.