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WASHINGTON — The U.S. Supreme Court late Friday refused to stop a lawsuit, scheduled to go to trial on Monday, over the Trump administration's plan to put a question about citizenship on the 2020 census form.

Eighteen states, several of the nation's largest cities, and immigrant rights groups brought the lawsuit, arguing that adding the question would making immigrants reluctant to respond to census takers, resulting in an undercount of that segment of the population.

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The Trump administration urged the court to put the case on hold, asserting that the challengers improperly sought to question Commerce Secretary Wilbur Ross about why he agreed to include the question on the form. Three of the Supreme Court's conservatives, Justices Clarence Thomas, Samuel Alito and Neil Gorsuch said they would have blocked the trial from proceeding until the issue of questioning Ross was resolved.

The census, conducted every 10 years, is required by the Constitution. The results determine the size of each state's delegation in the House of Representatives. Census figures are also used in calculating a state's share of funds under many federal grant programs.

The government said that questions about citizenship were included in nearly every census through 1950. For the 2020 census, the Trump administration said, the Justice Department asked that the question be added to provide more accurate information about the voting-age population.

The lawsuit, filed in July, said immigrant communities will resist cooperating with the census because of concern about how the federal government will use citizenship information.

"These concerns have been amplified by the anti-immigrant policies, actions, and rhetoric targeting immigrant communities of President Trump and this administration," the legal complaint said.

Postponing the trial would have reduced the chances of resolving the legal battle before printing of the census forms was to begin next year.