We suggest that Whites’ declining share of the U.S. population threatens their status as the most prototypical ethnic group in America. This prototypicality threat should lead to growing resistance toward diversity, motivated by the desire to reassert Whites’ standing as prototypical Americans. In Study 1, how dramatically Whites perceived their share of the population to decline predicted support for cultural assimilation, mediated by prototypicality threat (controlling for realistic and symbolic threat). This relationship held only among Whites who felt that ethnic groups differ in their prototypicality, not among those who saw all groups representing America equally. Study 2 experimentally manipulated exposure to demographic projections such that Whites who saw their group shrinking showed weaker diversity endorsement relative to those who believed their share to be stable, again mediated by prototypicality threat. These findings reveal Whites’ threatened prototypicality as a novel, emerging source of resistance toward diversity in 21st-century America.