An executive order President Trump issued this week to exclude undocumented immigrants from the official calculation of how many seats each state gets in Congress has raised questions about how the administration will adjust formal Census findings.

Few Census experts expect Trump's order to withstand legal scrutiny when it inevitably lands in court. Several states and groups have already issued litigious threats, pointing to a constitutional requirement that all persons be counted in the decennial Census.

But beyond the legal questions, those same experts had a more fundamental query: How does the administration plan to count those who are in the country illegally? 

It is not clear if any formal federal dataset covering undocumented workers exists. The Supreme Court last year blocked the administration from requiring the Census Bureau to ask whether respondents were citizens. Other agencies track immigrants in the country legally, though experts say there is no official way to track how many are in the country without authorization, or where those people live.

“There is no method that they currently have to do what he wants them to do. So how do you implement his order, if indeed it continues on? That’s really the paramount question,” said Kimball Brace, a Census and redistricting expert who runs the consulting firm Election Data Services.

She noted that various outside estimates have been released on the number of undocumented immigrants believed to be in the country, though those vary widely.

Trump’s executive order requires departments and federal agencies to share relevant information with the Census Bureau that could be used to help count the illegal alien population, a White House spokesperson said.

The executive order specifically directs the Department of Homeland Security’s Citizenship and Immigration Services, Immigration and Customs Enforcement, Customs and Border Patrol, the Centers for Medicare and Medicaid Services, the Social Security Administration and two offices within the State Department to provide data.

The White House spokesperson said the Census Bureau has been collecting records, though the spokesperson declined to elaborate on what datasets might be used.

The bureau directed questions to the White House. A spokeswoman for Commerce Department, which oversees the Census Bureau, did not respond to a request for comment.

“How are you going to sort out the illegal from the other citizens who are here legally? How are we going to get the estimates of the illegal population? So that seems to me to be highly challenging data collection,” said Michael McDonald, a political scientist and Census expert at the University of Florida. “I think it’s a nonstarter just because the data are not going to be available.”

Mere estimates about the undocumented population are hard to come by as well, a fact Trump’s own executive order acknowledges. It cites a 2018 study by Yale University researchers who estimated that between 16.2 million and 29.5 million people were in the United States illegally, figures that are far higher than other studies. A Pew Research Center study from 2016 estimated the undocumented population at 10.7 million.

Tinkering with official Census Bureau figures by eliminating an estimated number of undocumented immigrants would almost certainly change the allocation of U.S. House seats, bringing with it potentially significant ramifications.

States like California and Texas – which are home to 2.2 million and 1.6 million undocumented immigrants respectively, according to Pew’s figures – would stand to lose multiple seats in Congress. The undocumented population in Florida and New York are larger than the typical congressional district, too.

Other states would likely benefit, however, such as Alabama, West Virginia and Rhode Island, all of which are projected to lose a seat in the upcoming reapportionment process. Those states have tiny undocumented populations, meaning they would lose proportionally smaller fractions of their population.

The idea to exclude undocumented immigrants from the count came from a lawsuit filed two years ago by Alabama Attorney General Steve Marshall (R). Marshall’s suit alleged counting undocumented immigrants would cost Alabamians their right to equal representation.

Marshall praised Trump’s order when it was issued Tuesday.

“When the states’ Congressional seats and Electoral College votes are divided up, representation should be based on those people who reside in their states and this country lawfully. A contrary result would rob the State of Alabama and its legal residents of their rightful share of representation and undermine the rule of law,” Marshall said. “The state of Alabama is among several states that could lose a seat in the U.S. House of Representatives if illegal aliens are counted for apportionment in the 2020 Census.”

Some states have tiny margins for determining their number of congressional seats, meaning any effort to manipulate Census data could result in a shifted allocation. Brace’s most recent estimates show that Florida stands to gain its 29th seat — the second it would earn this redistricting cycle — by a margin of just 44,285 residents. Alabama, by contrast, would drop from seven seats to six by a margin of just 10,072 residents.

Illinois, New York, Texas and Montana appear to pick up the last congressional seats up for grabs, under the Commerce Department’s likely calculations. Minnesota, Ohio, California and Rhode Island fall just short of retaining their current total number of seats.

“We know the states that are on the cusp, and those are likely to be more impacted than not,” Brace said.

But those preliminary figures will not be finalized until the Census Bureau reports their final counts.

And, in an ironic twist, the coronavirus pandemic may alleviate Alabama’s complaint in the first place: Brace said new population estimates by the data giant ESRI shows Florida’s population growth has slowed sufficiently that it would no longer pick up a 29th district — and that Alabama would keep its seventh district.

Within states, Trump’s plan would grossly complicate the redistricting process. In a state like California, where the majority of the undocumented population lives in Los Angeles County, allocating fewer seats would mean larger congressional districts, leading to less representation for every region in the state — from Democratic Los Angeles to the Republican Inland Empire.

Trump’s plan would just as certainly draw a legal challenge from states that would stand to lose votes in the House. That is not unprecedented: A federal court ruled in 1998 against a Clinton administration effort to use sampling to get a different count in the 2000 cycle.

Even if the courts were to uphold Trump’s order, there is no guarantee that Congress — especially one under Democratic control — would ratify the Census Bureau’s figures.

“Even if the Trump administration presented the numbers to Congress, they don’t have to accept them,” McDonald said. 

— Brett Samuels and Rafael Bernal contributed to this report.