WASHINGTON — The Supreme Court ruled Thursday that the Trump administration cannot include a question about citizenship on the 2020 census form that goes to every U.S. household, giving a win to populous states that said it would discourage legal and illegal immigrants from responding and make the population count less accurate.

The ruling was a setback for the Trump administration’s tough position on immigration. It was also a surprise, because it appeared in April when the case was argued that the court’s five-member conservative majority was prepared to rule that Commerce Secretary Wilbur Ross acted within his authority to add the question. They seemed to agree with the Justice Department that Ross’ decision was a rational one, balancing the need for more information against concerns about accuracy. But in the decision Thursday, the court found that while the department of commerce has a right to reinstate the question, it did not provide an adequate justification for doing so.

A census is required every 10 years by the Constitution, and the results determine the size of each state’s congressional delegation. The data is also used to calculate a local government’s share of funds under many federal programs.

A total of 18 states, several of the nation’s largest cities and immigrant rights groups sued to block the question, saying it would make immigrants reluctant to respond to the census mailer. As a result, states with large immigrant communities could lose seats in Congress and suffer cuts in government aid programs, their lawsuit said.

Citizenship questions were asked during every census between 1820 and 1950. But from 1960 on, the government sent households a short form that contained only a few questions and did not inquire about citizenship. Both the Census Bureau and the groups behind the lawsuit agreed that the question will reduce the census response rate, especially in immigrant communities. By one government estimate, as many as 6.5 million people might not be counted.

Ross told Congress that he decided to add the question after receiving a letter from the Justice Department that said the citizenship data was needed to properly enforce federal voting laws. But he later admitted during a trial on the issue that he started thinking about the citizenship issue shortly after taking office and suggested that the Justice Department request it.

Led by New York, the states opposing the question also said Ross’ directive sidestepped the Census Bureau’s longstanding procedures for testing changes to the questionnaire in order to evaluate whether they would lead to an undercount. Because the citizenship question would depress minority responses, the challengers said, including it on the form would actually produce a less accurate count than leaving it off and using Social Security and IRS data to supplement the information gathered from the census form.

Five weeks after the case was argued in late April, the American Civil Liberties Union informed the court that it found evidence suggesting that the idea of including the question originated with an unpublished policy paper by a longtime adviser to the Republican Party. The government’s actual goal, the group said, was to dilute the voting power of minority communities.

But the Justice Department dismissed the claim as unfounded and said none of the officials responsible for adding the question had even heard of the policy document.