Commerce Secretary Wilbur Ross announced the addition of the census citizenship question in the spring. (Leah Millis/Reuters)

On the first day of a trial over adding a controversial citizenship question to the 2020 Census, a statistical data expert blasted the secretary of commerce’s assertion that the question would not harm the count.

D. Sunshine Hillygus, a professor at Duke University who studies survey methodology, repeatedly pointed to the Census Bureau’s own research to demonstrate that asking about an individual’s citizenship status would discourage participation among noncitizens and Latinos.

Hillygus cited the bureau’s research predicting that between about 5 and 12 percent of noncitizen households would decline to participate, based on the bureau’s analysis of the 2010 Census and long-form survey. She said the estimate was “conservative.”

“The Census Bureau completely agrees the addition of the citizenship question . . . is going to reduce the accuracy of the data,” said Hillygus, a former member of the Census Scientific Advisory Committee, which advises the Census Bureau on statistical data collection.

The debate over whether the question should be included in the constitutionally mandated decennial head count unfolded Monday at a federal court as the first of three trials opened on the issue. Dozens of states, cities and other groups have sued the administration, calling the question politically motivated and arguing it will depress participation in a process that helps determine apportionment of political power and federal funding.

When Secretary of Commerce Wilbur Ross announced the addition of the question in the spring, he told members of Congress it was in response to a request from the Justice Department to better enforce the Voting Rights Act.

But after six lawsuits were filed challenging the question, documents released by the government showed Ross had pushed for the question more actively and much earlier than he told Congress.

Plaintiffs in the New York lawsuit had asked for Ross to be questioned in the case. The Supreme Court last month temporarily blocked his deposition, but on Friday it denied the government’s request to delay the trial until a final ruling is reached.

The suits, which are being tried in New York, Maryland and California, are up against a deadline next summer, when the bureau is set to print the surveys. The case is likely to go to the Supreme Court.

Six former census directors and a Census Bureau internal analyst have said a citizenship question would harm the count. That analyst, John Aboud, was in the courtroom Monday, seated with Justice Department attorneys. He is expected to be the government’s sole witness.

On the witness stand, Hillygus referred to a recent Pew report showing that a majority of Latinos worry that they, a family member or a close friend could be deported. She also pointed to other research showing that the Trump administration’s crackdown on illegal immigration has led some immigrants to opt out of social programs such as food stamps, even if they are eligible.

Hillygus said the Census Bureau’s in-person follow-up process would do little to make up for an undercount.

She also said a memo in which Ross outlined his reasoning was mostly contradicted by what is known about survey science.

“The Census itself thinks it’s a bad idea, that’s what’s crazy about this,” she told The Washington Post. “Everyone except for Secretary Ross.”

A Commerce Department spokesman said: “Secretary Ross determined that reinstating the citizenship question on the decennial census and directing the Census Bureau to determine the best means to compare responses with administrative records will provide DOJ with the most complete and accurate voting age population by citizenship and race (CVAP) data in response to their request. Asking the citizenship question of 100 percent of the population gives each respondent the opportunity to provide an answer.”

New York Attorney General Barbara Underwood, who is leading the suit in New York, said that although Ross “obviously is the best source of what his real reason was . . . we have enough evidence here without his testimony.”

Immigrant rights groups held a rally outside the Manhattan courthouse, saying the citizenship question was part of a larger campaign by President Trump and others to exclude immigrants from the political process.

“Trump’s bigotry is baked into this ridiculous citizenship question,” said Steven Choi, executive director of the New York Immigration Coalition, which is among the plaintiffs in the suit. “It’s an obvious attempt to subvert the Constitution in order to deprive big, immigrant-rich states of federal dollars and political capital.”

The trial, before U.S. District Judge Jesse Furman, is expected to continue for two weeks.