Microcorporatism emphasizes organizing interest groups to advocate for their interests collectively in local participatory development initiatives. We study a microcorporatist approach to improve access to local services and resources for marginalized smallholder farmers. We leverage a randomized controlled trial in which a civil society organization ran a campaign with smallholder cooperatives and municipal authorities in municipalities in rural northern Paraguay. The study took place just after a 2016 law mandating participatory municipal planning. The campaign worked with cooperatives to improve the articulation of their corporate interests and with municipal authorities to host spaces for inclusive development deliberation. We find that smallholders, and particularly the leaders of the smallholders' collectives, substantially improve their standing vis-a-vis municipal authorities. Mayors' priorities for service provision are brought into alignment with smallholder priorities, as evidenced by the municipal development plans submitted to national authorities. Given the relatively short duration of the follow-up period, we do not find a measurable impact on smallholder income or agricultural output. Nonetheless, our findings show that the intervention can disrupt clientelistic dynamics, making rural development planning more inclusive and representative of the interests of the poor.