By then, the census was a modern and professional operation, with career staff members who distanced the agency, as best they could, from politics. With requests for more data tabulations streaming in from businesses and academics, the bureau had upped its game, collecting more information through sampling and applying more sophisticated statistical methods. When the bureau stopped asking every household about citizenship status, as it had on and off for more than a century, it did so for related technical reasons. To save money and increase efficiency, mailed questionnaires replaced door-to-door census-takers as the initial means of gathering data, and to make it easier and faster for people to fill out the form themselves, the number of questions was cut from 20 to five. A smaller statistical sample of the population was given a longer survey that asked about citizenship and birthplace, among other things.

Around the same time, in 1954, Congress sought to reassure the public about the security of census data by making it a crime, as opposed to a civil infraction, to disclose it without authorization, including to investigative agencies like the F.B.I. (and now ICE). The current acting Census Bureau director, Ron Jarmin — a career civil servant rather than a political appointee — has reaffirmed the bureau’s “absolute commitment” to confidentiality. But in November, an email surfaced in which the chief of staff at the time to John Gore, then the acting head of the civil rights division of the Justice Department, wrote, “I don’t think we want to say too much” about the legal protections for census data in case “related issues come up later for renewed debate.”

In January, John Abowd, the Census Bureau’s chief scientist, wrote to Wilbur Ross warning that reinserting the citizenship question would be very costly, harm the quality of the count and produce “substantially less accurate citizenship status data than are available from administrative sources.” The bureau had not had time to field-test the question for the census, but there were plenty of reasons to predict it would have a negative impact in the current political climate.

In 2017, researchers at the bureau conducted focus groups on the coming census and found an “unprecedented” level of concern about giving the government identifying information. People of Hispanic origin and Middle Eastern and North African descent spontaneously said they wouldn’t fill out the census for fear of “registering” relatives and other household members with the government. They spoke of ICE and “the Muslim ban.” They said they feared that filling out the census would result in people being arrested and deported. In a Census Bureau survey last spring of 17,500 people across the country, about one-quarter were “very” or “extremely concerned” that their answers would not be kept confidential and could be used against them. The rate of concern shot into the 40 percent range among people born outside the United States and low in English proficiency.

Ross testified to Congress in March that he went ahead with the citizenship question “solely” because the Justice Department “initiated the request” for the purpose of enforcing the Voting Rights Act, which relies on survey data the Census Bureau collects. In the 53 years since the act’s passage, the Justice Department had never previously made such a request, and the government could not identify any voting rights case it had been unable to bring for lack of adequate data.

By law, a judge can block a decision by a federal official like Ross by finding, among other things, that it is “arbitrary and capricious,” which can mean that the decision is not supported by the stated rationale. If a government action that appears to be neutral has a greater impact on a particular group based on its race, ethnicity, religion or national origin, it may violate the Constitution’s guarantee of equal protection — but only if the government is found to have acted at least in part because of the “adverse effects upon an identifiable group,” in the words of the Supreme Court. It’s hard to prove discriminatory intent, but the plaintiffs in the census case are trying.

When Ross’s emails related to the census were released last summer before the trial, they showed that Stephen K. Bannon, the anti-immigrant former White House strategist, asked Ross soon after he took office if Ross would talk to Kris Kobach, then the Kansas secretary of state (and later a failed candidate for governor), about adding a citizenship question to the census. In the spring of 2017, Ross directed his staff to ask the Justice Department to request citizenship data from the census. That May, he emailed his aides that he was “mystified that nothing have [sic] been done.”