The Supreme Court on Thursday blocked the Trump administration from adding a question about citizenship to the 2020 census for now, and asked the Commerce Department to offer a better explanation for why it made the move.

The opinion pointed to questions raised by civil rights groups and Democratic-led states, which argued that the reason Commerce Secretary Wilbur Ross gave was a pretext that would actually skew the count in a way that would help Republicans.

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Ross has contended that he added the question to help the Justice Department enforce a voting rights law. The challengers say the question would count up to 9 million few people in immigrant communities, where households with noncitizens will be less likely to respond.

Chief Justice John G. Roberts Jr., who joined the liberal wing of the court for the majority, wrote that the voting rights enforcement rationale — the sole stated reason — “seems to have been contrived” and was “more of a distraction.”

“Altogether, the evidence tells a story that does not match the explanation the Secretary gave for his decision,” Roberts wrote.

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The court stopped short of invalidating the decision to add the citizenship question, but it leaves open the question of how the Commerce Department will come up with another explanation.

“We do not hold that the agency decision here was substantively invalid,” Roberts wrote. “But agencies must pursue their goals reasonably.”

The government’s task now is made harder with the disclosure of last-minute information that the challengers say shows how a Republican strategist served as a source of the question and pushed it as a way to eventually draw legislative maps favoring Republicans.

That strategist, Thomas Hofeller, has since died, but questions about his contacts with the Census Bureau and the Justice Department are part of a House Oversight and Reform Committee probe and will be reviewed by lower courts in the coming weeks.

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The decision also leaves open questions about the government’s timeline for completing the census. There is some uncertainty about the final deadline for starting to print census forms. While the government has claimed a June 30 deadline, the Fourth Circuit noted testimony from a government witness that put the deadline at the end of October.

Justice Clarence Thomas, in a dissent joined by Justices Neil M. Gorsuch and Brett M. Kavanaugh, wrote that Ross’ decision “was legally sound and a reasoned exercise of his broad discretion.”

Thomas wrote that this holding could transform administrative law “if taken seriously,” and lead political opponents of executive action to generate controversy with accusations of pretext, deceit, and illicit moves.

Friday’s opinion is one of the most significant for members of Congress during the current Supreme Court term. The census results determine how many House seats each state gets and affect how states redraw congressional districts. They also are used to distribute billions of dollars from federal programs that are based on population count to states and local governments.

The House, appearing at oral arguments in April, told the justices that the Commerce Department acted improperly and the addition of a citizenship question would reduce the actual count of people in the country — a task the Constitution gives to Congress.

The Oversight and Reform Committee continues to investigate Ross’ decision to add the citizenship question. Democrats claim the administration has stonewalled the investigation, and voted to recommend the House hold Attorney General William Barr and Ross in contempt of Congress for defying subpoenas in the probe.

The challengers had argued that the question would deter households with noncitizens from participating in the census for fear that their information would be shared with other parts of the government — immigration authorities, for example. And a short count of noncitizens would harm communities with large numbers of immigrants and ultimately benefit Republicans.

“This case has never been about a line on a form,” said Dale Ho, the director of the ACLU’s Voting Rights Project who argued the Supreme Court case. “It is about whether everyone in America counts. This ruling means they do.”

In recent decades, the question has been part of the long-form census questionnaire, which goes to about 1 in 6 households, but not on the short form that most households get. And the citizenship question is on the annual American Community Survey questionnaire sent to approximately one in 38 households since that survey began in 2005.

Michael Macagnone contributed to this report.