The Supreme Court on Thursday delivered a setback to the Trump administration's plans to add a citizenship question to the 2020 census, effectively blocking the addition of the question for now.
In an opinion authored by Chief Justice John Roberts, the court questioned the administration's reasoning for adding the question and ordered the case to be reconsidered by a lower court.
The Census Bureau has said it faces a July 1 printing deadline for the forms, raising questions about whether the government will be able to add the question even if it ultimately prevails. The deadline may turn out to be later. A government witness has said, in testimony quoted by a federal appeals court this week, the real deadline may be as late as October 31.
On Thursday, the Census Bureau said only that the court's decision was under review. In a series of tweets, Trump said he wanted to delay the census and had sought legal advice about whether that would be possible.
Those who challenged the question declared victory.
"This case has never been about a line on a form," Dale Ho, an attorney for the American Civil Liberties Union who argued the case, said in a statement. "It is about whether everyone in America counts. This ruling means they do."
In a section of the opinion joined by the court's four liberals, Roberts wrote that Commerce Secretary Wilbur Ross's explanation for adding the question was "contrived."
"We do not hold that the agency decision here was substantively invalid," Roberts wrote. "But agencies must pursue their goals reasonably. Reasoned decisionmaking under the Administrative Procedure Act calls for an explanation for agency action. What was provided here was more of a distraction."
The court's holding, which was decided in relevant part by Roberts and the court's liberal wing, was slammed by Roberts's fellow conservatives.
Justice Clarence Thomas, in a separate opinion joined by fellow conservative Justices Neil Gorsuch and Brett Kavanaugh, said that part of Roberts's opinion "reflects an unprecedented departure from our deferential review of discretionary agency decisions."
"And, if taken seriously as a rule of decision, this holding would transform administrative law," Thomas wrote.
Justice Samuel Alito, another conservative, wrote that the court "set a dangerous precedent" that could serve as "a license for widespread judicial inquiry into the motivations of Executive Branch officials."
The debate over the addition of the question was fierce, as the the census can have wide-ranging effects that last a decade.
The Constitution requires the government to conduct a census every 10 years, and the results of the survey are used to allocate billions of dollars in federal funding. The census is also used to determine representation in the House and the Electoral College.
Critics of the citizenship question argue that asking it would result in less accurate data and disproportionately harm cities and states with large immigrant populations, as well as immigrants themselves. The question has not been posed to all U.S. households since 1950.
If included on the census, one government analysis determined that as many as 6.5 million fewer people may respond. As a result, California, Texas, Arizona, Florida, New York and Illinois all face serious risk of losing a seat in the House, one federal court found.
"Today's decision calls into question the Trump Administration's dubious reasoning for adding the citizenship question to the 2020 Census, but we also know that the fight must go on," California's Attorney General Xavier Becerra, whose office filed a brief arguing against the addition of the question, said in a statement.
The decision was handed down on the final opinion day of the Supreme Court's term. It was released shortly after a 5-4 decision along partisan lines that held that federal courts may not block gerrymandered congressional districts.
The case is known as Department of Commerce v. New York, No. 18-966.
The Trump administration acknowledged in court that the inclusion of the citizenship question could make the survey less accurate. But the Commerce Department argued that the question would enable the government to better enforce certain provisions of the Voting Rights Act.
That rationale did not pass muster before three federal courts, which each blocked the addition of the question. Those courts, in New York, Maryland and California, found that the rationale offered by the Commerce Department was a pretext.
The official justification for the question came under further scrutiny in recent weeks after new documents surfaced that provided new evidence of a political motive behind the addition of the question. The documents were obtained by the government watchdog group Common Cause, and provided to the justices.
One of the documents the group uncovered was an unpublished 2015 study by an influential Republican redistricting expert, who found that the addition of the citizenship question could benefit "Republicans and Non-Hispanic Whites" if the data were used in redistricting.
The study's author, Thomas Hofeller, appeared to help ghostwrite a 2017 draft letter from the Justice Department to the Commerce Department that provided the Voting Rights Act rationale for the citizenship question, the documents show.
After Hofeller's death in August, his estranged daughter discovered hard drives in her father's home and turned them over to Common Cause.
"The Supreme Court saw through the explanations by the Commerce Department as pure pretext. The last-minute effort to add the question was clearly a cover-up to mask their true motives — to rig redistricting for partisan and racial gain," Kathay Feng, Common Cause's director of redistricting and representation, said in a statement after the opinion was released.
The Trump administration denied that Hofeller played any role in the Justice Department's request for the citizenship question.
-- CNBC's John Schoen contributed to this report.