While the 2020 census does not include a citizenship question, the president argued that excluding "illegal aliens" from the count "reflects a better understanding of the Constitution and is consistent with the principles of our representative democracy."
"My Administration will not support giving congressional representation to aliens who enter or remain in the country unlawfully, because doing so would create perverse incentives and undermine our system of government," Trump wrote in a memorandum. "Just as we do not give political power to people who are here temporarily, we should not give political power to people who should not be here at all."
That statement was met with stiff opposition from Democrats, including House Speaker Nancy Pelosi, who signaled she would challenge the effort in court. Pelosi said that the memo is "unconstitutional," adding that the Constitution makes clear that the "actual enumeration" of the "who number of persons" is used for congressional apportionment.
"Trump's unlawful effort is designed to again inject fear and distrust into vulnerable and traditionally undercounted communities, while sowing chaos with the Census," Pelosi, D-Calif., wrote in a statement. "The House of Representatives will vigorously contest the President's unconstitutional and unlawful attempt to impair the Census. We will be vigilant to ensure a full, fair and accurate Census and continue working to guarantee maximum participation and inclusion so that every person in every community is counted."
Absent the citizenship question on the census, it is unclear how the Trump administration would accurately determine how many people to exclude from the data that contributes to reapportionment of Congressional Districts, or how many seats each state gets in the U.S. House of Representatives.
During a news conference in the Rose Garden last week, the president acknowledged uncertainty regarding the total number of people living in the United States.
"Nobody can give the exact count. We're trying to get an exact count," Trump said July 14. "But you have, over the years, many illegals who have come into the country, so it depends on how you want to count it. But you could say 325 to 350 million people."
Pelosi said the Constitution unambiguously requires an "actual enumeration" of the "whole numbers of persons" for the population count and congressional apportionment.
Amid the COVID-19 pandemic, and given the rhetoric of last year's debate over the citizenship question, the census is already facing an uphill battle with a lot of communities.
Opponents of the presidential memorandum argue there is much more than a congressional representation issue at hand, and regardless of if it pans out, impacts may still be felt.
"Trump wins just by further connecting immigration and the census in the public mind," said Michael Kagan, a UNLV Immigration Clinic director and a plaintiff in a Maryland case that produced one of the final injunctions on the census citizenship question last summer. "If that leads some immigrant families to fear the census, that accomplishes the same goal of undercounting the population of diverse communities."
Congressional districts are apportioned according to data derived from census, which counts every person in a household, as well as the homeless population.
Last year, the Supreme Court blocked the administration from adding a census question about citizenship, but left the door open for another attempt to track non-citizens.
Trump called the decision "ridiculous," and said he wasn't backing down. Then, in July 2019, the White House issued an executive order to gather government data on non-citizens, including those living in the country legally.
The president's memo says he claims his authority from an executive order signed on July 11, 2019, which asks the Secretary of Commerce to use methodology from a report setting Census Residence Criteria and Residence Situations "to allow the Secretary to obtain accurate data on the number of citizens, non-citizens, and illegal aliens in the country."
That methodology is set forth in the Final 2020 Census Residence Criteria and Residence Situations, 83 Fed. Reg. 5525, a 168-page document posted to the federal register on Feb. 8, 2018.
That document states that apportionment "is based on the resident population, plus a count of overseas federal employees, for each of the 50 states."
It also states that the Census Bureau "will retain the proposed residence situation guidance for foreign citizens in the United States."
"Foreign citizens are considered to be 'living' in the United States if, at the time of the census, they are living and sleeping most of the time at a residence in the United States," it reads.
The move has energized many congressional Democrats, while Republicans have largely abstained from reacting to the memorandum.
"President Trump is a bigot and a racist, and he's using the levers of power to harm the people he hates," Rep. Norma Torres, D-Calif., wrote in a statement. "To go uncounted in the census is to be unseen by our government -- there are no support systems in place for people our government doesn't know exist. It's been a long time since this nation's highest office fell to someone this low, and we can only hope that it never happens again once the shadow he casts on this nation is finally gone."
"This executive order is unconstitutional and only serves the purpose to please Trump's campaign and stoke anti-immigrant reactions," Rep. Raúl Grijalva, D-Ariz., noted. "Right before the election, Trump and Stephen Miller are flailing for something to hold on to."
"Undocumented immigrants live in our communities and must be counted as persons -- not only for the sake of accuracy, but for the recognition of their dignity as human beings and as our neighbors," House Majority Leader Steny Hoyer, D-Md., argued. "We must not allow our Constitution to be further undermined by this president who has demonstrated no regard for its articles and disdain for the rule of law it protects."
The American Civil Liberties Union also announced it is prepared to fight the order in court.
"The Constitution requires that everyone in the U.S. be counted in the census," said Dale Ho, director of the ACLU's Voting Rights Project. "President Trump can't pick and choose. He tried to add a citizenship question to the census and lost in the Supreme Court. His latest attempt to weaponize the census for an attack on immigrant communities will be found unconstitutional. We'll see him in court, and win, again."
ABC News' Ben Gittleson, Quinn Owen and Victoria Moll Ramirez contributed to this report.