This Article intervenes in a burgeoning literature on “administrative constitutionalism,” the phenomenon of federal agencies — rather than courts — assuming significant responsibility for elaborating the meaning of the U.S. Constitution. Drawing on original historical research, I document and analyze what I call “administrative equal protection”: interpretations of the Fourteenth Amendment’s Equal Protection Clause in a key federal agency at a time when the Clause’s meaning was fiercely contested. These interpretations are particularly important because of their interplay with cooperative federalism — specifically, with states’ ability to exercise their traditional police power after accepting federal money. The Article’s argument is based ...