Many theorists assume that the openness of borders and the inclusiveness of citizenship trade off. Yet, there is no consistent empirical evidence for this negative relationship. In this paper, I propose the boundary politics framework to explain the association between immigration and citizenship regimes in democratic states. I argue that when immigration is not politicized, immigration and citizenship politics are not correlated because they follow distinct logics. But when politicized, immigration and citizenship politics become part of a common cosmopolitan-nativist conflict about boundary-making, which manifests in a positive correlation between immigration openness and citizenship inclusiveness. Nativist parties long-term legislative power in politicized settings should further induce a common restrictive logic, making boundary regimes more closed and exclusive. I test these propositions using quantitative analyses across 23 democracies 1980-2010 along with some case study evidence. The results support but also qualify the new framework and bear implications for long-standing normative and empirical debates on boundary regime making.