Recent scholarship on whiteness typically views white identity as a stable category in American politics, yet this obscures its historically contested boundaries. As that work has shown, however, explicit racial pride is expressed by only a relatively low proportion of white Americans. As is the case with other racial groups, shared racial categories frequently mask important differences within groups, particularly when it comes to ethnic heritage. As a result, white racial and ethnic identity in America cannot be construed monolithically or hegemonically. We argue that the boundaries of “white” as a racial identity germinated in a haphazard and fluid manner throughout the nineteenth and twentieth centuries. Therefore, attachment to this identity relative to others that might be more stable such as ancestral country of origin, religion, etc., is on average relatively weak. We begin this argument by documenting the nature of whiteness in American politics, and the fluid and dynamic nature of how immigrants of varying ethnic backgrounds are variously classified as “white” or not. Using historical examples from immigrant groups such as Italians, Irish, and Poles, we illustrate the contested and changing definitions of whiteness.