How do members of dominant groups, like White people in the United States, react when their privileged social status is threatened, for example, by the prospect of numeric decline? Prior studies identify two sets of reactions: (1) White people identify more strongly with ingroup members, and (2) they withhold material and symbolic resources from outgroup members. This study explores another possibility: White people may alter the boundary around Whiteness by redefining the criteria for membership. I use an original survey experiment to examine how demographic threat affects how White people in the United States classify people who are ambiguously White, and specifically people who are ambiguously White or Latino. The results reveal that White people are less—not more—likely to classify people who are ambiguously White or Latino as “White” under threat. The results contribute to a growing literature on the racial classification of multiracial and racially ambiguous people that has previously ignored ambiguity around the Latino category. They also speak to an active debate about demographic projections and the classification decisions on which they rest.