Interest groups representing the marginalized regularly neglect advocacy on behalf of their most vulnerable constituents—those with intersectional disadvantage. Yet, they claim that such advocacy is central to their missions. I argue that interest groups representing women, people of color, Native nations, and the poor strategically conduct intersectional advocacy through coalitional lobbying. I test this claim using a new dataset of cosignature patterns within public comments on proposed federal agency rules submitted by a set of such groups between 2004 and 2014. I find that these groups are significantly more likely to pursue intersectional advocacy in coalitions but that coalition work, alone, does not relate to influential intersectional advocacy. Rather, it is particular coalition characteristics, including organizational diversity and financial capacity, that predict such influence. I conclude that collaborative lobbying is an effective tactic for mediating representational bias in interest group advocacy and promoting more pluralistic administrative policy making.