The primary purpose is to determine, based on population, how many seats each state will have in the House of Representatives — and by extension, how many votes in the Electoral College. But census data is used for a great many other purposes as well, including the allocation of about $900 billion in federal spending each year. That money helps pay for everything from public schools and Medicaid benefits to law enforcement and highway repairs. State and local governments use the data in similar ways, including setting the boundaries of legislative districts.

Why is asking about citizenship such a big deal?

The Justice Department says it wants the question included in the census because it needs to have a better idea of how many Americans are eligible to vote. The government says it needs that information to enforce the Voting Rights Act, which bars discrimination against racial or language minority groups in the conduct of elections.

That may seem uncontroversial, but critics say it is far from the whole story. They dismiss the voting-rights argument, saying the government’s current estimate of the number of voting-age American citizens is sufficient for that purpose. They say the citizenship question is actually a central element of a Republican strategy to try to shift political boundaries to the party’s advantage when the states begin using the new census to redraw their district maps in 2021.

Wilbur Ross, the commerce secretary and the architect of the revised 2020 census form, has said that he ordered that the citizenship question be added to the standard census form solely in response to a December 2017 request from the Justice Department. But three federal trial judges ruled that the evidence in the record demonstrated that Mr. Ross was not telling the truth. He had decided long before to add the question, the judges found, and then pressed the Justice Department to supply a rationale.

Opponents say the citizenship question is intended to frighten noncitizens away from participating in the census, whether they are in the country legally or not. The American Civil Liberties Union says that it would make the count less accurate, and would have the effect of diverting federal money and political power away from states and cities where larger numbers of noncitizens tend to live, and into the hands of rural areas.