On June 2, 1924, the U.S. government unilaterally extended U.S. citizenship to Native Americans by passing the Indian Citizenship Act over the objection of some Native Nations. As dual citizens of their tribes and the United States, members of federally-recognized tribes should have been able to register and participate in both nontribal (U.S.) elections and tribal elections.
Sourced from https://narf.org/anniversary-indian-citizenship-act