But in their appeal in Department of Commerce v. New York, Trump administration advocates cite the federal law that gives the Commerce secretary broad leeway to conduct "in such form and content as he may determine."
The high court agreed to bypass the appellate courts and hear the case now because the government plans to begin printing census forms in the summer.
The court's four liberals, led by Justice Sonia Sotomayor, dominated much of the argument, insisting that adding a citizenship question would lead to an undercount.
"There's no doubt people will respond less," she said.
Justice Elena Kagan joined her in arguing that Ross' reasons for adding the question appear "contrived."
"I went searching in the record for a reason" for adding the new question, she said, since most of the evidence showed it would lead to an inaccurate tally. "I didn't find a reason."
But the court's conservatives were equally firm in arguing that the decision was a reasonable one and should be upheld.
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Justices Neil M. Gorsuch and Brett M. Kavanaugh pointed out that citizenship questions were included on the census through most of the nation's history and that most of the other leading nations in the world ask a similar question of their residents.
The law calls for the court to be "deferential" to the decision of the Commerce secretary, Kavanaugh said.
Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr. commented that Ross' decision seemed reasonable, and the courts had no basis for invalidating it.
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