Trump wants to distill a massive trove of data across seven government agencies - and possibly across 50 states. It's far from clear how such varying systems can be mined, combined and compared.
He directed the Commerce Department, which manages the census, to form a working group.
"The logistical barriers are significant, if not insurmountable," said Paul Light, a senior fellow of Governance Studies at New York University with a long history of research in government reform. "The federal government does not invest, and hasn't been investing for a long time, in the kind of data systems and recruitment of experts that this kind of database construction would require."
Trump says he aims to answer how many people are here illegally, though there already are recent estimates , and possibly use such information to divvy up congressional seats based on citizenship. It's also a way for Trump to show his base that he's not backing down (even as he's had to back down) from a battle over the question on his signature topic, immigration.
The administration faced challenges last year when it was tasked by a federal judge with quickly creating a system to track migrant families that had been separated by immigration officials. They found agency systems weren't compatible.
"Information-sharing is not a habit of federal agencies," Light said.
Trump's plan is aimed at yet-again circumventing legal challenges on an immigration related matter, as courts have barred him from inquiring about citizenship on the 2020 census. But it could spark further legal action, depending on what his administration intends to do with the citizenship information.
His executive order announced Thursday requires highly detailed information, including national-level files of all lawful permanent residents, Customs and Border arrival and departure data and Social Security Administration master beneficiary records. Plus information on Medicaid and children's health systems and refugee and asylum visas.
The order states that "generating accurate data concerning the total number of citizens, non-citizens and illegal aliens in the country has nothing to do with enforcing immigration laws against particular individuals," and that information would be used "solely to produce statistics" and would not be used to "bring immigration enforcement actions against particular individuals."
Dale Ho, the director of the ACLU's Voting Rights Project who argued the citizenship question case at the Supreme Court, said the main privacy concern now would be disclosure of individuals' citizenship status.
Federal law bars the Census Bureau from disclosing an individual's responses to the census. But Ho said that if the bureau can produce citizenship information in small geographical bites, it could inadvertently expose a person's citizenship status.
The bureau has methods in place that are designed to prevent such disclosures, but "we don't know enough yet to know the answers," Ho said.
In March, the Associated Press reported that even before the outcome of the census question litigation, U.S. Citizenship and Immigration Services, which maintains some of the requested data, had been working on a data-sharing agreement that would give census access to names, addresses, birth dates and places, as well as Social Security numbers and alien registration numbers.
The possibilities worried immigrant rights advocates, especially given Trump's hardline stance on immigration.
Samantha Artiga, a Medicaid expert with the nonpartisan Kaiser Family Foundation, said she is concerned that Trump's directive will discourage some immigrants from applying for health benefits they'd be entitled to.
"It is likely that this policy will further enhance already heightened fears among families about applying for Medicaid or the Children's Health Insurance Program for lawfully present immigrants or citizen children in immigrant families, potentially leading to fall-offs in coverage," she said.
But to some degree, Trump's directive reflects what was already being put into place before the controversy about a citizenship question on the census. The Census Bureau had stressed that it could produce better citizenship data without adding the question and had recommended combining information from the annual American Community Survey with records held by other federal agencies that already include citizenship records. The survey polls 3.5 million U.S. households and includes questions about citizenship.
"It's a retreat back to what he should have done from the beginning," said Kenneth Prewitt, a former Census Bureau director.
Transferring the data from other agencies to the Census Bureau is not necessarily difficult, but some, like Customs arrivals data, contain hundreds of millions of entries and it will take time to compile, maybe years. Getting the information to match up with census data will be the main challenge.
Prewitt said government records tend to be highly accurate for some purposes and less so for others. It's essential for the Social Security Administration, for instance, to know the age of Americans accurately, but it isn't as concerned with addresses.
According to a 2018 report, the Census Bureau already has access to data from the IRS, Social Security, the Department of Housing and Urban Development, the Postal Service, the Selective Service System, the Centers for Medicare and Medicaid Services and the Indian Health Service. The agency also gets data from some states that administer federal programs such as food stamps, Temporary Assistance for Needy Families and the Women, Infants, and Children nutrition program.
Virtually all federal social programs are open only to citizens or to immigrants who have been lawfully present for at least five years.
"I think the executive order will just hurry up negotiations about data-sharing that are already in the works," said Julia Gelatt, a senior policy analyst with the nonpartisan Migration Policy Institute think tank.
Commerce Secretary Wilbur Ross, who is in charge of the Census Bureau, insisted on adding the citizenship question and a legal challenge ensued, ending with a ruling by the Supreme Court temporarily barring its inclusion on the grounds that the government's justification was insufficient.
He had offered multiple explanations for why he believed the question was necessary to include in the once-a-decade population count that determines the allocation of seats in the House of Representatives for the next 10 years and the distribution of some $675 billion in federal spending.
Even after the Supreme Court ruled against him, Trump insisted he was pushing forward, contradicting government lawyers and Ross, who had conceded the case was closed, as well as the Census Bureau, which had started printing the 2020 questionnaire without the controversial query.
Trump toyed with the idea of halting the constitutionally-mandated survey entirely while the court battle played out. But by Thursday evening, he gave up on including the question in the census and announced the executive order.
Associated Press Writer Geoff Mulvihill in Trenton, New Jersey contributed to this report.
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