This has been black letter law since the 3⁄5 compromise was overturned by the fourteenth amendment of the US Constitution in 1868:

A federal court on Thursday rejected President Trump’s order to exclude unauthorized immigrants from population counts that will be used next year to reallocate seats in the House of Representatives, ruling that it was so obviously illegal that a lawsuit challenging the order need not go to a trial.

The court, a three-judge panel in Federal District Court in Manhattan, said Mr. Trump’s proposal exceeded his authority under federal laws governing the census and reapportionment. The specially convened panel said there was no need to consider a second claim in the lawsuit that the president’s order violated the Constitution’s requirement to base apportionment of the House on “the whole number of persons in each state.”

“The merits of the parties’ dispute are not particularly close or complicated,” the judges wrote in granting summary judgment to the plaintiffs, a view that was broadly shared by legal scholars. Two of the judges, Richard C. Wesley and Peter W. Hall, were named to the bench by President George W. Bush. The third, Jesse M. Furman, was nominated by President Barack Obama.

The case involved lawsuits brought by two sets of plaintiffs, one a group of state and local governments and the United States Conference of Mayors, and the second a coalition of advocacy groups and other nongovernmental organizations. The groups argued that Mr. Trump’s order would cause some of them to lose representation in the House and would damage all of them by leading to a less accurate census.

Since the first census was taken in 1790, the number of seats each state holds in the House of Representatives has been based on counts of everyone living in the United States regardless of citizenship or legal status, except for slaves and “Indians not taxed” during the nation’s early years. Former slaves gained citizenship in 1866; all Native Americans did in 1924, although they were counted in regular censuses beginning in 1900.

Mr. Trump tried to scrap that process in July, telling the Commerce Department in a memorandum that the Census Bureau should produce two population counts — one the same as those taken every decade, and the other an estimate of the number of unauthorized immigrants living in each state.

I do not expect that the Supreme Court to rule on this before on the election.