The Trump White House wants to add a citizenship question to the census, and it falls within the authority of the executive branch to make the change. Yet the administration cannot just attach the question because it says so. It must provide an adequate explanation, as the law requires. That was the responsible position expressed by Chief Justice John Roberts for a 5-4 majority of the Supreme Court last week.

For now, there will be no citizenship question on the census conducted next year. The high court has given the president and his team time to try again at offering sufficient reason for taking the step.

So far, Wilbur Ross, the secretary of commerce, who oversees the census, has contended the question would help to improve enforcement of the Voting Rights Act. He says he ordered the inclusion of the question in response to a request from the Justice Department in late 2017. Yet three federal judges found Ross isn’t telling the truth. The record shows he was determined from the time he entered office to add the question. More, he approached the Justice Department in advocating for the addition.

Thus, the chief justice concludes the administration’s explanation “seems to have been contrived.” In a stinging passage, Roberts adds that the executive must “offer genuine justifications for important decisions, reasons that can be scrutinized by courts and the interested public. Accepting contrived decisions would defeat the purpose of the enterprise. If judicial review is to be more than an empty ritual, it must demand something better than the explanation offered for the action taken in this case.”

The relevant law is the federal Administrative Procedure Act. It requires “genuine justifications,” or showing in this matter how in a concrete way enforcement of voting rights would be advanced. The chief justice now finds “the evidence tells a story that does not match the explanation the secretary gave for his decision.”

What has surfaced since the court heard oral arguments in April reinforces what the chief justice describes as a “disconnect.” Documents have emerged from the files of the late Thomas Hofeller, an influential Republican strategist. They include a report he wrote in 2015 arguing that the addition of a citizenship question would give Republicans a significant advantage in drawing new legislative district lines. His words show up in an eventual Justice Department letter arguing for the citizenship question.

Since 1950, the census has not included a citizenship question in the forms sent to each household. At the time, census experts voiced concern about the question depressing participation and resulting in an inaccurate count. They have the same worry today, one projection putting the undercount at 6.5 million people.

The constitutional requirement calls for an “actual enumeration” of persons living in the country. That becomes hard to obtain when many Hispanics and other minorities, here legally or not, are wary about filling out the forms. Some experts worry that harm already has been done, the controversy surrounding the question reinforced by the president’s divisiveness and fear-mongering about immigration.

Secretary Ross claims any such undercount would be more than offset by the gains in enforcing voting rights. Now he gets another chance to show, precisely, how that would be the outcome, that his reasoning isn’t mere cover for something politically sinister. The census involves more than the redrawing of congressional districts and the apportioning of Electoral College votes. It goes to the way federal dollars are spent, from highways to public schools, roughly $900 billion a year in all.

This is an exercise to get right, and kept removed from becoming a vehicle for rank partisan gain.