Dred Scott Decision, 1856

The words "people of the United States" and "citizens" are synonymous terms, and mean the same thing. They both describe the political body who ... form the sovereignty, and who hold the power and conduct the Government through their representatives.... The question before us is, whether the class of persons described in the plea in abatement compose a portion of this people, and are constituent members of this sovereignty? We think they are not, and that they are not included, and were not intended to be included, under the word "citizens" in the Constitution, and can therefore claim none of the rights and privileges which that instrument provides for and secures to citizens of the United States. On the contrary, they were at that time considered as a subordinate and inferior class of beings, who had been subjugated by the dominant race, and, whether emancipated or not, yet remained subject to their authority, and had no rights or privileges but such as those who held the power and the Government might choose to grant them. 
 (Dred Scott v. Sandford, 60 U.S. 393, 1856)