WASHINGTON (Reuters) - The U.S. Supreme Court on Thursday handed President Donald Trump a stinging defeat, blocking his contentious citizenship question planned for the 2020 census because officials gave a “contrived” rationale and prompting Trump to suggest an extraordinary delay in the constitutionally mandated population count.

Conservative Chief Justice John Roberts wrote the 5-4 ruling, joined by the court’s four liberals, that will make it difficult for the Trump administration to add the query even if officials offer a new explanation for its need, with the clock ticking toward the deadline for printing the census forms.

The court upheld a key part of a federal judge’s January decision barring the question in a victory for a group of states including New York and immigrant rights organizations that challenged the legality of the administration’s plan.

Critics have called the citizenship question a Republican ploy to scare immigrants into not taking part in the decennial population count and engineer an undercount in Democratic-leaning areas with high immigrant and Latino populations. That would benefit non-Hispanic whites and help Trump’s fellow Republicans gain seats in the U.S. House of Representatives and state legislatures, the critics said.

Trump said he is exploring whether the census, which the U.S. Constitution requires be carried out every 10 years, can be delayed.

“I have asked the lawyers if they can delay the Census, no matter how long, until the United States Supreme Court is given additional information from which it can make a final and decisive decision on this very critical matter,” Trump wrote on Twitter.

The census launch date - April 1 - is written into a federal law called the Census Act, known as Title 13. To delay the 2020 census without running afoul of federal law would require Congress to change the law, according to Margo Anderson, a U.S. census historian and professor at the University of Wisconsin at Milwaukee. Delaying the census by executive action would be unprecedented, Anderson added.

Trump’s administration has told the courts that its rationale for adding the question was to better enforce a law that protects the voting rights of racial minorities. Critics called that rationale a pretext, with the Supreme Court’s majority embracing that theory.

Roberts said that under a U.S. law called the Administrative Procedure Act, the federal government is required to give a reasoned explanation for its actions. Roberts said the sole stated rationale - enforcement of the Voting Right Act - “seems to have been contrived” and was “more of a distraction.”

“We are presented, in other words, with an explanation for agency action that is incongruent with what the record reveals about the agency’s priorities and decision-making process,” Roberts wrote.

“Accepting contrived reasons would defeat the purpose of the enterprise,” Roberts added.

The court ruled against the challengers in a separate 5-4 vote, with all the conservative justices in the majority, that the Constitution does not in theory prevent the administration or a future one from adding a citizenship question.

As part of the ruling issued on the last day of the court’s current term, the justices sent the issue back to the Commerce Department for it to decide whether to provide a different rationale for requiring people taking part in the census to declare whether they are citizens.

Such a determination from a federal agency normally would take weeks or months. The administration previously said census forms need to be printed in the coming days.

“There is really no time,” said Dale Ho, a lawyer with the American Civil Liberties Union, one of the challengers.

Jason Torchinsky, a lawyer and Republican redistricting strategist, said the deadline could be extended to the fall, enabling the Supreme Court to review the new decision.

“They don’t need to redo the entire administrative process, they just need to better justify what they did, in a way that can satisfy the Supreme Court,” Torchinsky said.

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Justice Department spokeswoman Kelly Laco said the administration is disappointed by the Supreme Court’s decision.

The census is used to allot seats in the U.S. House and distribute some $800 billion in federal funds. Opponents have said the question would instill fear in immigrant households that the information would be shared with law enforcement, deterring them from taking part.

Further muddying the waters, there is also ongoing litigation in lower courts over recently unearthed evidence that the challengers have said reveals an illegal discriminatory motive by the administration for adding the question, which the high court could yet weigh in on.

In a dissenting opinion, conservative Justice Clarence Thomas wrote that adding a citizenship question is “legally sound” and described the ruling against Trump as “an aberration.”

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Citizenship has not been asked of all households since the 1950 census, featuring since then only on questionnaires sent to a smaller subset of the population.

SETBACK FOR TRUMP

The ruling marked the first major setback for Trump in a ruling in a case argued at the Supreme Court, although both of Trump’s appointees, Justices Neil Gorsuch and Brett Kavanaugh, voted in favor of his administration. Roberts was joined by liberal Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan.

The Supreme Court had handed Trump some major victories since he took office in 2017, in particular a June 2018 ruling upholding his travel ban targeting people from several Muslim-majority countries. The court in January also let Trump’s policy barring many transgender people from the U.S. military go into effect.

Manhattan-based U.S. District Judge Jesse Furman ruled on Jan. 15 that the Commerce Department’s decision to add the question violated the Administrative Procedure Act. Federal judges in Maryland and California also have issued rulings to block the question.

Furman said the evidence showed that Commerce Secretary Wilbur Ross concealed his true motives for adding the question and that he and his aides had convinced the Justice Department to request a citizenship query.

The Census Bureau’s own experts estimated that households corresponding to 6.5 million people would not respond to the census if the citizenship question were asked.

While only U.S. citizens can vote, non-citizens comprise an estimated 7 percent of the population.

Evidence surfaced in May that the challengers said showed that the administration’s plan to add a citizenship question was intended to discriminate against racial minorities.

Documents created by Republican strategist Thomas Hofeller, who died last year, showed he was instrumental behind the scenes in instigating the addition of the question. He was an expert in drawing electoral district boundaries that maximize Republican chances of winning congressional elections.

Hofeller concluded in a 2015 study that asking census respondents whether they are American citizens “would clearly be a disadvantage to the Democrats” and “advantageous to Republicans and Non-Hispanic Whites” in redrawing electoral districts based on census data.

Hofeller suggested the voting rights rationale in the newly disclosed documents.

The Trump administration called the newly surfaced evidence “conspiracy theory.”

For a graphic on major Supreme Court rulings, click tmsnrt.rs/2V2T0Uf For a graphic on the U.S. census, click here