President Donald Trump announced Thursday that he was ordering a new United States census that would exclude undocumented immigrants, arguing that it's unconstitutional to count them despite courts regularly holding that the census must count everyone, regardless of immigration status.

With estimates on the undocumented population varying widely, from around 11 million to as high as 20 million, just how big is the U.S. population, and just how much impact are undocumented migrants having on the wider, legal population?

"Congressional representation is a zero-sum proposition. When one state gains a seat in Congress due to the presences of illegal aliens, American citizens in another state necessarily lose representation," Ira Mehlman, media director at the anti-immigration Federation for American Immigration Reform (FAIR), told Newsweek when asked about Trump's plan.

"By some estimates, California's large illegal alien population resulted in that state getting four to five more representatives than it would have if illegal aliens were excluded for this purpose. The system actually rewards states that enact policies that draw illegal aliens."

How Big Is The US Population?

Over the last two years, estimates for the overall U.S. population have hovered above 331 million, based off the 2020 census, with the latest estimate from the U.S. Census Bureau in its American Community Survey putting the number at 334,914,896.

That figure was broken down by native-born population (85.7 percent) and foreign-born population (14.3 percent), with nearly two-thirds of the latter having entered the U.S. before 2010.

Out of the 47.8 million foreign-born U.S. residents, 24.9 million were naturalized U.S. citizens, while the rest were not, but their legal status was not provided.

With immigration at the heart of the national political conversation in recent years – following unprecedented crossing numbers at the U.S.-Mexico border, programs to allow for legal migration from the Biden administration, and calls for mass deportations – much has been written about what the data on population numbers really means.

In March, the non-partisan Migration Policy Institute (MPI) stated that immigration had accounted for the entirety of the U.S.' population growth in a single year for the first time, amid falling American-born birthrates.

More recently, the right-leaning Center for Immigration Studies found that the foreign-born population grew by more than 300 percent over the past 46 years in 26 states.

Trump, and the MAGA movement, are focused on illegal immigrants and the concern that including them in census data has a negative impact on natural-born and naturalized citizens, particularly when it comes to representation.

Undocumented Immigrants' Impact on Congress

"These numbers matter enormously for apportionment—states like California, Texas, and Florida have substantial undocumented populations that currently contribute to their congressional representation," Gil Guerra, immigration policy analyst at the Niskanen Center, told Newsweek.

The difference between the legal and illegal immigrant population is important. The Trump administration is specifically looking to avoid counting the undocumented population by the time the next census is scheduled to take place in 2030, leaving those with legal status still counted among the U.S. population.

The U.S. Constitution requires a census every decade that counts the "whole number of persons in each state." This language has long been interpreted by the courts to include all residents, regardless of citizenship or immigration status.

Currently, the wide range of estimates – including from Trump himself during the 2024 election campaign – make it difficult to fully know the impact of undocumented immigrants on population counts.

If the figure is around 11 million — as estimated by the Biden administration in 2022 — then the share of the total U.S. population would be between 3 and 4 percent, per Pew Research Center and MPI. Conservative and anti-immigration groups put the estimate at far higher.

FAIR put the figure of undocumented immigrants in the U.S. at around 18 million. Under that number, the undocumented share of the U.S. population would be about 5 percent.

"The inclusion of illegal aliens in the Census not only impacts apportionment of representation, but potentially the outcome of national elections," Mehlman said, arguing for a census count that strips out undocumented residents.

"Each state's electoral vote count is based on the number of seats it holds in the House. Thus, the current system of counting illegal aliens for congressional apportionment could determine the outcome of a close presidential election."

David Bier, director of immigration studies at the libertarian Cato Institute, told Newsweek that making such a change to the population count would likely have little effect on the distribution of seats in Congress, "with very little partisan effect on net."

"The plan is blatantly unconstitutional," Bier said. "There is not even the slightest ambiguity here. He is effectively setting aside the Constitution based on a partisan conspiracy theory."

The Census Bureau has already said it is making plans for its next count in 2030, stating on its website that it is working on an "initial, high-level design", but making the kind of change the president wants to see would require a constitutional amendment.

"It seems to me that the plan will likely get mired in legal challenges," Guerra of the Niskanen Center, told Newsweek, pointing to a similar attempt Trump made toward the end of his first term in 2020.

"The most likely outcome is protracted litigation that could drag past the 2026 midterms, making the exercise moot for redistricting purposes. Moreover, this could backfire if the resulting undercount affects red states with growing immigrant populations like Texas and Florida."