The sociology of knowledge, derived from research on the hard sciences, overlooks the potential for outsiders to determine the content of knowledge within professional disciplines. Using the case of the Sally Hemings affair, I introduce the concept of "knowledge advocacy" to analyze how outside groups shape historical knowledge. The Hemings controversy involved not only historical evidence, but also the understanding of objectivity in historical research. Unfolding against the backdrop of the professionalization of history, outside advocates successfully challenged the discipline's understanding of "objectivity as neutrality" eventually embracing "objectivity as the scientific method" in their appeal to DNA testing. This study illuminates the strategic interplay between professional historians and outsiders engaged in knowledge advocacy, the role of objectivity in this struggle, and the potential vulnerability a discipline faces when the ideal of objectivity is compromised. (Contains 5 notes.)