Do inclusive societies need closed borders? The classical view in political theory and much empirical research argue that openness in immigration and inclusiveness in citizenship indeed trade off. This paper reformulates the underlying puzzle in a general way: How are Immigration Regime Openness (IRO) and Citizenship Regime Inclusiveness (CRI) associated? I claim is that this association depends upon the politicization of immigration and its changing meaning for the transformation of party politics across time. When immigration is politicized in more recent historical periods, the politics of immigration and citizenship align along a new cultural cleavage that pits nativists seeking closure against cosmopolitans seeking openness in both territorial and membership boundary-making. This is boundary politics. It should manifest in a positive correlation between IRO and CRI. I argue further that immigration and citizenship regimes are relatively closed and exclusive when nativist party power is strong and relatively open and inclusive when it is weak. Contexts with low politicization lack boundary politics and should instead exhibit no systematic correlation between IRO and CRI. I test this framework by combining quantitative analyses across 23 democracies 1980-2010 with evidence from case studies of five reforms across three countries. Corroborating most propositions of the boundary politics framework, the findings not only advance our empirical understanding of the context-dependent links between immigration and citizenship politics, but also bear important implications for the long-standing normative debate on the topic. Most importantly, they can rejuvenate theoretical efforts going beyond the widespread trade-off assumption, which this paper falsifies.