The NAACP said it is planning to file a lawsuit against the Census Bureau, the secretary of commerce and President Trump to force a more accurate count of minority populations such as those residing in Prince George’s County, Md., which had one of the highest undercounts nationwide in the last census.

Calling the government’s preparation for the 2020 count “conspicuously deficient,” the lawsuit will allege that the census violates a constitutional mandate to count all the people in the country and disproportionately harms African American and Hispanic populations.

The civil rights organization’s national headquarters, along with its Prince George’s branch and the county, said it plans to file the lawsuit, which also names the bureau director and the United States, on Wednesday.

The lawsuit was in the works before Monday night’s announcement by Commerce Secretary Wilbur Ross that the 2020 decennial count would include a question about citizenship for the first time since 1950. Critics of the question say it will scare many immigrants and their family members from completing the survey, leading to a less accurate and more costly count and skew toward Republican-friendly redistricting.

Prince George’s County, home to a large population of minorities, suffered the largest net undercount of any county in Maryland and one of the largest undercounts in the United States for counties of 100,000 or more residents, the lawsuit alleges.

The lawsuit, whose lead counsel is the Yale Law School Rule of Law Clinic, charges that the bureau’s plan to use digital technology for the count for the first time while canceling field tests due to funding shortages, will result in “a massive undercount of communities of color.”

The decennial count determines the distribution of congressional seats, affects the shape of districts and helps decide the flow of hundreds of billions of dollars a year in federal funding. It is also used by businesses, civic organizations and others whose work relies on demographic data.

The census typically overcounts non-Hispanic whites and homeowners, and undercounts minorities, immigrants, children and the poor.

A government study found that the 2010 Census had a net undercount of 2.1 percent of African Americans and 1.5 percent of Hispanic Americans, totaling 1.5 million black and Hispanic residents — enough to fill two congressional districts, according to the suit. The overcount was 0.6 percent for homeowners and 0.8 percent for non-Hispanic whites.

The organization is filing the suit in advance of the decennial in an attempt to force the bureau to take measures to avoid an undercount, said Bradford Berry, general counsel for the NAACP.

“To some extent, our country has become numb to this undercount problem,” Berry said. “It’s a very serious problem that deserves attention. After the fact is too late. . . . We want the court to instruct the Census Bureau to develop a targeted, well-thought-out plan to address the undercount and to refrain from taking measures that are likely to increase the undercount and design measures that are likely to decrease the undercount.”

The 2020 count has been plagued by a funding shortage, which was partially mitigated last week when the spending bill for 2018 included more money than expected for the census. Berry said that was a step in the right direction but did not ensure the bureau would use the money to prevent an undercount or guarantee adequate funding in 2019 and 2020.

Prince George’s County, home to around 900,000 residents, suffered a 2.3 percent net undercount in the 2010 Census and has been consistently underfunded because of past census undercounts, the suit alleges.

“An accurate census count is critical to the federal funding, political representation, and operations of Prince George’s County,” said County Executive Rushern L. Baker III (D) in a statement announcing the suit. “We must not be undercounted again like we have been over the past 30 years. . . . A vast majority of the residents of Prince George’s County are members of this nation’s historically disenfranchised populations. We cannot let this continue in 2018, 2020, or any year moving forward.”

Bob Ross, president of the NAACP Prince George’s County Branch and a named plaintiff in the lawsuit, added: “When the Census Bureau undercounts my community, we lose political power, and fewer of our federal tax dollars end up coming home to fix our roads, run our schools, and fund our federal programs. . . . We felt these effects in the aftermath of the 2010 Census, and all signs indicate that the 2020 Census will be even worse.”

In October, the NAACP filed suit under the Freedom of Information of Act to compel the Commerce Department to produce documents about its preparations for the 2020 Census, especially regarding how it will communicate information to minority and low-income communities. That suit is pending.