In political organizing, groups use deliberation to scope projects in which they will collaboratively design political actions and organizations. Despite its importance, we lack a model for teaching learners to scope highly open-ended political projects through deliberation. We designed, implemented, and evaluated deliberation-based learning (DBL), a novel model of learning environments that combines support for iterative design and deliberation, in a university design course. We found the learning environment supported students to choose political issues, form teams, and scope detailed project proposals from scratch, by completing iterations of proposing ideas, raising questions, suggesting improvements, planning to-dos, seeking information, and updating their proposals. This study contributes DBL, a novel, empirically grounded model of learning environments for scoping design projects through deliberation, which can be further refined through multi-case studies across contexts. By understanding DBL, learning scientists can engage students in political organizing in their communities—a key to sustaining democracy.