As congressional redistricting battles rage from Texas to California, President Trump is promising to raise the stakes: a new approach to the 2030 Census. His proposal would include only legal United States residents (and, thus, potential voters) in the census population count. 

As he put it on Truth Social, in reference to his election victory based on his immigration restriction platform: “People who are in our Country illegally WILL NOT BE COUNTED IN THE CENSUS.”

The change would likely reduce the congressional delegations in major blue states, New York and California among them, where the foreign-born are disproportionately found and counted for purposes of representation in the House. The president can well be said to be zeroing in on the districts of leading Democrats like Representatives Alexandria Ocasio-Cortez and Hakeem Jeffries.

There is no doubt that the number of blue state members of Congress is swelled by the constitutional requirement, in Article I , Section 2, that the decennial census include an “actual enumeration” of the population. That means that all residents — legal or not — must be considered in a state’s population count to determine the extent of its congressional representation and electoral votes.  

The number of congressional districts is fixed at 435 — which means that the way the pie is divided matters greatly. There’s no way to know exactly how many of those the Census includes are not here legally, but we do know that some districts have large numbers of the foreign-born and many of those are likely illegals. 

The census has estimated that there are 53 million foreign-born residents in America. The Center for Immigration Studies estimates there to be 14 million — or 26 percent — who are here illegally. 

Striking such a large group from the population count would have a profound impact on congressional districts. It would reduce the resident count in Ms. Ocasio-Cortez’s district by 80,000, and in Mr. Jeffries’s district by 74,000 — raising the likelihood that theirs or other New York City Democratic districts would have to be combined. 

The same prospect could be faced in California, where 27 percent of the population is foreign-born, and Illinois, where 14 percent of the population was born outside America. Texas, too, has a high immigrant population, constituting 14 percent of the populace, but, as in other states, it’s concentrated in major cities, where Democrats dominate.  

Many of the legal immigrants, what’s more, are not citizens. But since, by law, each congressional district must have roughly the same number of residents (around 740,000), that means that some members of Congress represent far more citizens — and voters — than others. The Center for Immigration Studies reports that members of Congress from districts with fewer immigrants represent far more citizens — and voters — than those with more.

Not only have Democrats, as the White House clearly understands, benefited from the sheer population count of illegal immigrants, members such as AOC also benefit by being able to safely ignore the preferences of a larger number of the residents of their districts. As a practical matter, that means lower campaign costs, such as postage for mailings.

The outstanding question about the president’s order, however, is whether it is legal. In 2019, the Supreme Court ruled that the census could not include a question about citizenship.  It was a five-to-four decision, however, and based on the court majority’s view that the government’s stated rationale — enforcing the Voting Rights Act — was not its true underlying motivation. 

A court case that focused clearly on whether the “enumeration” clause required those here illegally to be counted might lead to a different outcome. Moreover, Mr. Trump is seeking to use his authority to change not the law but the protocol of census-takers. Rolling that back would require a new court challenge — to halt the next census before it’s taken. 

Democrats are already apoplectic about the effort of Texas Republicans to redraw the state’s congressional districts to favor their party. If Mr. Trump is successful in excluding illegal immigrants from the population count, the impact on congressional districting would be far greater. Look out, AOC.