LOS ANGELES — In a squat office building not far from downtown, Esperanza Guevara is getting ready to look for people who might not want to be found. And her job could get a lot harder.

The immigrant-rights activist is leading a drive to reach tens of thousands of people who entered the U.S. illegally and persuade them to participate in the 2020 census, the government’s once-a-decade count of the population.

The Trump administration’s plan to use the census to inquire about each person’s citizenship has sent a chill through immigrant communities. Guevara and others fear the question could discourage participation and, by some estimates, leave millions uncounted across the country.

Such concerns are concentrated in Democratic-led states with large immigrant populations. An inaccurate count could have real-world consequences, since billions in federal dollars and seats in Congress are allocated according to population.

In immigrant communities often wary of government, a question about citizenship status will make people “less likely to fill out the census form or even answer the door when someone comes knocking,” said Guevara, who works for the Coalition for Humane Immigrant Rights of Los Angeles.

Those concerns have been heightened by Trump’s slashing rhetoric toward immigrants and by fears that census information could be used to find and deport people.

“Their first thought is, ‘Is this information going to be used against me?’” Guevara said, standing near rows of computers that will be staffed by volunteers trying to connect with prospective census participants.

Census Bureau chief Ron Jarmin said the agency is legally barred from sharing its information with law enforcement agencies, adding: “We are committed to ensuring that the data we collect are always protected.”

The U.S. Supreme Court is weighing a legal challenge seeking to strike the citizenship question from the census form. During oral arguments last week, the court’s conservative majority appeared ready to allow the question.

The Trump administration has argued that it has wide discretion in designing the questionnaire and that the citizenship question is clearly constitutional because it has been asked before — most recently, 1950 — and continues to be used on smaller, annual population surveys.

The Public Policy Institute of California has said that failure to accurately tally immigrants and other hard-to-reach groups could lead to an undercount of 1.6 million people, or roughly 4 percent of the state’s population. That would be enough to cost California one of its 53 House seats.