Every ten years, the U.S. government undertakes a census or count of everyone living in the United States. The Constitution mandates this in Article 1, Section 2, and established the first census would take place in 1790 and every ten years thereafter. The census language is straightforward and explicitly ties the census to representation in the republic government. The 14th Amendment, ratified in 1868, modified the census language and the definition of citizenship to include everyone born within the United States regardless of origin, making freed slaves citizens for the first time. The 14th also made children born within the national borders citizens, whether their parents were citizens or present in the country legally or not. The latter is likely an unintended consequence, as post-Civil War America was preoccupied with Reconstruction and, for the Republicans, bringing the slaves the party was founded to free toward the rights of citizenship.
Both the original language in Article 1, Section 2 and the 14th Amendment tie the census directly to representation in the U.S. House of Representatives. This is important to understanding the purpose of asking about citizenship on the census.
Inquiring about citizenship on the census goes back to at least 1800 and Thomas Jefferson. The founder, author of the Declaration of Independence and president-to-be wanted the census to ask about the “respective numbers of native citizens, citizens of foreign birth, and of aliens...for the purpose of more exactly distinguishing the increase of population by birth and immigration.” In 1820 the census began asking the questions President Jefferson wanted answered.