President Trump’s opponents asked a federal judge late Monday to order Commerce Secretary Wilbur Ross to sit for a sworn deposition to answer questions about why he decided to ask about citizenship on the 2020 census, saying there are key matters only he can settle.

Subjecting Mr. Ross to a deposition is extraordinary, but New York Attorney General Barbara D. Underwood, the chief plaintiff, said there’s no other way to learn about whether the White House pressured him to add the citizenship question.

She is particularly eager to learn who Mr. Ross was referring to when he said “other senior administration officials” first raised the need for the question.

The deposition could have serious consequences.

Already the judge in the case has expressed skepticism about the question, saying that while asking about citizenship isn’t in and of itself illegal, if the question was added for the wrong reasons it could be.

He said there’s evidence of “bad faith” already on the part of the Trump administration, after the administration had first said the question grew out of a request from the Justice Department, then later admitted Mr. Ross had been contemplating it well before that request came in.

The Census Bureau is part of the Commerce Department.

The Justice Department says it wants the question added back into the full 2020 count — it had been part of counts up through 1950, and remains part of the continual smaller American Community Survey — in order to get good information to enforce voting rights laws.

But evidence that pressure from the White House and from key Trump allies on immigration has led the president’s opponents to argue the question is intended to scare immigrants and others from completing the census, thus skewing its count.

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