A seven-day trial began Monday before a federal judge in San Francisco on a challenge to the U.S. Commerce Department's plan to include a citizenship question in the 2020 census.
U.S. District Judge Richard Seeborg will decide the case without a jury.
He is presiding over two lawsuits, one filed by the state of California, later joined by Oakland, Fremont and four other cities and counties, and a second suit filed by San Jose and the Black Alliance for Just Immigration.
The cases are among several filed against Commerce Secretary Wilbur Ross in federal courts around the nation.
A lawsuit filed by more than 30 states, cities and counties led by the state of New York went to trial before a federal judge in New York City in November and is now awaiting a decision from that judge. San Francisco and Monterey County joined that lawsuit.
Another lawsuit filed by individual residents of Maryland, Arizona and California will go to trial before a U.S. judge in Baltimore on Jan. 22.
The census question would ask people whether they or members of their household are citizens.
The states and cities that sued claim the use of the question violates the U.S. Constitution's requirement of an "actual enumeration" of the population every 10 years because non-citizen immigrants will be deterred from participating in the census.
They say the reduced participation will unfairly result in decreased federal funding and congressional representation, which are based on the census information.
The Justice Department, defending Ross and the Commerce Department, contends the question is reasonable and answers will be subject to strict confidentiality requirements.
The first witness before Seeborg today was University of Chicago public policy professor Colm O'Muircheartaigh, an expert on survey methodology.
He cited a number of studies that he said show that non-citizens asked to answer a citizenship question feel that "this is a threatening activity" and may refuse to participate in the census.
One study showed that people responding to the census on behalf of a household may decide not to mention other household members who are non-citizens, he told the judge.
"This is very disturbing, because the census depends on getting a full response from the household," the professor said.