(Reuters) - The U.S. Supreme Court on Friday rejected a request by President Donald Trump’s administration to halt a trial set to begin on Monday that will test the legality of the government’s contentious decision to ask people taking part in the 2020 national census whether they are citizens.

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The justices announced the action in a one-sentence order. Three conservative justices - Clarence Thomas, Samuel Alito and Neil Gorsuch - said they would have granted the request for an indefinite postponement of the trial. Trump’s new appointee to the nine-member court, Brett Kavanaugh, did not state publicly how he voted on the matter.

The decision benefits the 18 states challenging the census citizenship question in part because such a postponement could have made it impossible to resolve the dispute before census forms are printed starting next year.

The administration had argued there should be no trial until the justices rule on a fight over evidence. That dispute includes whether Commerce Secretary Wilbur Ross, whose department oversees the U.S. Census Bureau, should be forced to answer questions under oath about his motivations for the politically charged decision.

Opponents of the citizenship question have said it would deter people in immigrant communities from participating in the census, disproportionately affecting Democratic-leaning states’ electoral representation and federal funding by undercounting the number of residents.

Amy Spitalnick, a spokeswoman for New York Attorney General Barbara Underwood, who is among the state officials suing the administration, said the Trump administration has tried “every trick in the book” to block the case.

“You really have to wonder what they’re trying to hide. We welcome the Supreme Court’s decision and look forward to making our case in court as we fight for a full and fair census,” Spitalnick said.

The U.S. Justice Department, which is defending the administration, did not immediately respond to a request for comment.

The trial, in federal court in New York City, was scheduled in a pair of lawsuits. The first, spearheaded by Democratic officials, was brought by 18 states as well as a number of cities and counties. The other was filed by several immigrant rights groups that accused the administration of discrimination against non-white immigrants.

The administration, in explaining the citizenship question, has said more precise citizenship data is needed to better enforce a voting rights law in order to protect minorities.

There has not been a census question about citizenship status since 1950. The plaintiffs have said that in recent decades Census Bureau officials have opposed adding a citizenship question because of the risk of driving down response rates and undercounting the U.S. population.

Manhattan U.S. District Judge Jesse Furman ruled in July that the plaintiffs “plausibly allege” that Ross’s decision was motivated by discrimination.

Furman ruled in September that Ross must face a deposition by lawyers for the states because his “intent and credibility are directly at issue” in the lawsuit..

The Supreme Court later blocked that order.

The U.S. Constitution mandates a census every 10 years. It is used in the allocation of seats in Congress and the distribution of billions of dollars in federal funds.