Legislative allies are widely recognized as key to social movement success, but the emergence of their alliance with activists remains understudied. This article proposes a strategic approach to this phenomenon based on the cases of the environmental, labor, and LGBT+ movements in Chile and their allied legislators. According to this approach, an alliance emerges due to two necessary conditions. Movement organizations must display tactical capacity, which signals their adaptability and competence to participate in Congress. And a socially skilled leadership creates the trust required for movement leaders and legislators to cooperate during the lawmaking process. This approach emphasizes that alliances emerge from activists’ strategic efforts to build a social tie, whose effectiveness is mediated by legislators’ expectations and congressional norms. By specifying the strategic dimension of an alliance, this study highlights the capacity of activists to foster cooperative relations with state actors.