This article focuses on a non-State player created in the 1970s in order to defend mathematicians persecuted for their political opinions and commitments under non democratic regimes both of the left and right. While it was built incrementally as an internationalized advocacy network located in several countries and claiming itself from the defence of human rights, the International Committee of Mathematicians also succeeded in gathering around it a heterogeneous and variable coalition of allies, and even in occupying a pivotal position within a network that extended beyond the mathematical milieu itself. Based on a survey conducted mainly on its French component, the article aims to map out this progressive construction of a broaden coalition of allies of the committee and the intersectoral dimension of its action. It retraces the trajectories of some of the main cause entrepreneurs involved and the dynamic of this extension to committees anchored in other scientific disciplines, associative circles, trade unions, professional milieus such as lawyers and psychiatrists, as well as to partisan and State players. This broader configuration within which the French branch of the International Committee of Mathematicians itself was located provided it with varied types of resources, skills, knowledge and know-how and helped diversifying its forms of intervention, while increasing their impact. It did not exclude, however, distinction from, and even competition with, other players.