A coalition of 20 states and 15 cities and counties filed suit Friday to block President Donald Trump's memo directing undocumented immigrants be excluded from the 2020 census count for purposes of deciding how many members of Congress are apportioned to each state, calling it "unlawful."

The suit, filed in New York federal court, charges that Trump's directive shows "blatant disregard of an unambiguous constitutional command" the 14th Amendment's directive that Representatives shall be apportioned among the several States according to their respective numbers, counting the whole number of persons in each State."

"For 150 years since the United States recognized the whole personhood of those formerly bound in slavery the unambiguous requirement that all persons be counted for apportionment purposes, regardless of immigration status, has been respected by every executive official, every cabinet officer, and every President. Until now," the suit said.

The Trump memo, signed earlier this week, said it will be the "policy of the United States to exclude from the apportionment base aliens who are not in a lawful immigration status under the Immigration and Nationality Act."

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It directs Commerce Secretary Wilbur Ross, who oversees the census, to provide the president with data about the number of people who are undocumented so that when census officials present Trump with the final count, he can exclude them from the population totals used to determine how many House seats each state will have.

The lawsuit questions how the directive could feasibly be carried out, noting the administration "cannot reliably exclude undocumented immigrants from the apportionment count" because there are no accurate estimates of the undocumented population on a state-by-state basis. Any effort to remove undocumented immigrants from the census count using unreliable data is "arbitrary and capricious" and in violation of the Administrative Procedure Act, the suit said.

If the Trump move is successful, it could have a major impact on states with large numbers of undocumented residents, such as New York and California. A lower population count in the census would mean fewer seats in Congress, and potentially fewer federal dollars as well, the suit noted.

New York Attorney General Letitia James, who's leading the lawsuit, called the proclamation "the latest in a long list of anti-immigrant actions" by the president.

No one ceases to be a person because they lack documentation, which is why we filed this lawsuit," James said.

Another of the plaintiffs in the case, Michigan Attorney General Dana Nessel, called Trump's actions "a clear breach of the U.S. Constitution and a blatant attempt to politicize a non-partisan process by targeting certain groups of people and advancing a biased agenda.

The suit seeks an order declaring the president's proclamation unlawful.

The Department of Justice, which would defend the case in court, did not immediately respond to an email for comment.

The administration tried last year to add a citizenship question to the census for the first time in 60 years, but the U.S. Supreme Court blocked the move.

Dareh Gregorian is a politics reporter for NBC News.

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'Unlawful': Coalition of states sues Trump over bid to omit undocumented immigrants from census - NBC News