The Supreme Court has effectively blocked the Trump administration from adding a citizenship question to the 2020 census , giving a partial victory to states and civil rights groups who said the question would jeopardize what is perhaps the most crucial information the U.S. government collects.

The case, Department of Commerce v. New York , arose after a number of states, cities and advocacy groups sued the Trump administration and claimed the process the administration used to add the citizenship question ran afoul of federal law. Commerce Secretary Wilbur Ross , who oversees the census, was set on adding the question and ignored clear evidence that showed it was a bad idea. U.S. District Judge Jesse Furman agreed with the plaintiffs in January, ruling the Trump administration had violated the Administrative Procedure Act and thus could not add the question. Two other judges later also struck down the citizenship question.

In its ruling, the Supreme Court said that the Department of Commerce did not provide an adequate explanation for its decision and sent the case back to a lower court for further review.

The surprising ruling would still allow for a citizenship question in future years. But it may make it impossible for a citizenship question to get on the 2020 census. The Census Bureau has repeatedly said it needs a final answer from the courts on the citizenship question by June 30 so it can start printing the census forms.

President Donald Trump tweeted Thursday afternoon that hes asking government lawyers if they can delay the Census to accommodate a citizenship question.

Given the time, given the level of mendacity that weve seen to date, its very difficult for me to imagine that succeeding, said Dale Ho, a lawyer at the American Civil Liberties Union who represented a group of plaintiffs challenging the citizenship question. You cant just come up with a new reason after saying for months, there was no other reason.

The Justice Department did not immediately address whether it would continue to push for a citizenship question on the census. We are disappointed by the Supreme Courts decision today. The Department of Justice will continue to defend this Administrations lawful exercises of executive power, Kelly Laco, a DOJ spokeswoman, said in a statement.

Although the Supreme Court said it did not disagree with the merits of adding a citizenship question, it said the Trump administration had not adequately explained its reasoning for adding the question. The Trump administration could attempt to further explain its reasoning, but it is unlikely the administration would be able to offer such an explanation in just a few days to meet the Census Bureaus deadline.

It is undisputed that adding the citizenship question would have made it more difficult for the Trump administration to fulfill its constitutional mandate to count every living person in the country. A Census Bureau analysis estimated that adding the question would likely cause an additional 2 million households , roughly 6.5 million people, not to respond to the survey on their own.

An inaccurate decennial census would have huge consequences. Businesses rely on census data to make informed decisions about their products, and the federal government uses it to distribute about $880 billion in federal funds each year. States also use census data to redraw the boundaries for electoral districts once every 10 years. Because of the sensitivity of census data, the U.S. Census Bureau carefully tests and calibrates each question before adding it. But Ross decided to add the question in 2018 without testing it at all.

The Trump administration argued that the benefits of adding a citizenship question outweighed the risk that fewer people would be counted. The Justice Department formally requested in 2017 that the administration add the question so that it could better enforce the Voting Rights Act. Even though the Census Bureau advised Ross there were other ways to get the data the Justice Department wanted, Ross ultimately decided to add the question. The Trump administration also says it can mitigate any drop in self-response by extensively following up with people.

The plaintiffs in the case, along with many civil rights groups, say the Trump administrations justification for adding a citizenship question is a lie. The decennial census has not asked about citizenship since 1950, and the Voting Rights Act was passed in 1965. Justice Department officials have also failed to name voting rights cases it could have brought or won if it had had better citizenship data. Emails made public as part of the litigation showed Ross was interested in adding a citizenship question long before the Justice Department requested it. Commerce Department officials also solicited the Justice Department to formally make the request, the emails showed.

Lawyers in the case also say they recently uncovered evidence the decision to add the question was motivated by a desire to change the way legislative districts are drawn. They say they have files from a deceased GOP mapmaker showing he played a role in getting the question on the census. (The Justice Department denies his involvement.) The same mapmaker had authored a 2015 study showing that adding a citizenship question was necessary to allow officials to draw districts based on the voting-age citizen population, not the total population. Drawing districts that way, he wrote in the study, would benefit white people and the Republican Party .

In the opinion for the majority, Chief Justice John Roberts said the evidence tells a story that doesnt match up with Ross explanation for why the question belonged on the census. While Ross maintains he was simply acting on a routine data request from another agency, the materials before us indicate that Commerce went to great lengths to elicit the request from DOJ (or any other willing agency), Roberts wrote.

The VRA enforcement rationale the sole stated reason seems to have been contrived, he added.

Justice Clarence Thomas, in a separate opinion, defended Ross decision. Unable to identify any legal problem with the Secretarys reasoning, the Court imputes one by concluding that he must not be telling the truth, he wrote. Justices Neil Gorsuch and Brett Kavanaugh joined Thomas.

Justice Stephen Breyer, joined by Justices Ruth Bader Ginsburg, Elena Kagan and Sonia Sotomayor, also wrote a separate opinion saying they believed there was already sufficient evidence to show Ross decision was illegal.

The Secretarys failure to consider this evidence that adding the question would harm the census count in the interest of obtaining less accurate citizenship data provides a sufficient basis for setting the decision aside. But there is more. The reason that the Secretary provided for needing more accurate citizenship information in the first place to help the DOJ enforce the Voting Rights Act is unconvincing, Breyer wrote.

Vanita Gupta, president and CEO of the Leadership Conference on Civil and Human Rights, said even if the citizenship question was not ultimately on the census, the possibility of it had already scared immigrants.

We know that even the specter of the citizenship question has intensified this climate of fear. And even in the best of times without a citizenship question, the work to get to an accurate count is one of the most challenging operations in peacetime America, she said. After todays decision, which we all hail as a huge win for democracy in this country, we also know our work isnt over.

Hayley Miller and Gavrielle Jacobovitz contributed reporting.