Across Western democracies, education levels are predictive of immigration attitudes and voting for new left or far right parties. What explains education-based political divides? This article proposes that social closure of higher- and lower-educated citizens strengthens and reinforces differences in political attitudes and voting between them. Using social network data from the Netherlands, and ESS data, we show that large proportions of higher- and lower-educated citizens report no close relationships with different education levels. Network education levels, in turn, are predictive of immigration attitudes and voting behaviour. Difference-in-differences models show that a change in network education levels is associated with change in these outcomes. Our findings contribute to literatures on educational divides and peer effects. Moreover, they support an interpretation of political competition on the universalist-particularist dimension as durably rooted in social structure. Sizable, distinct and insulated educational groups can crystallize contemporary divides and predictably shape political reality.