“If this policy is ever actually implemented,” Mr. Ho said, “we’ll be right back in court challenging it.”
The practical effect of the ruling may be limited because Census Bureau officials have said they may not be able to produce data on unauthorized immigrants before Mr. Trump leaves office. When the case was argued last month, Jeffrey B. Wall, the acting solicitor general, acknowledged that the situation was fluid and that the bureau may not be able to identify many immigrants who are in the United States without authorization.
By law, the Commerce Department is required to supply census information to the president by the end of the year, but it may not be able to meet that deadline. Mr. Wall indicated that the Census Bureau does have good data on the tens of thousands of unauthorized immigrants held by Immigration and Customs Enforcement, but that number is almost certainly too small to change apportionment.
He was less certain that the bureau could match records concerning people subject to orders of removal, young immigrants known as Dreamers or other categories of unauthorized immigrants.
The Constitution requires congressional districts to be apportioned “counting the whole number of persons in each state,” using information from the census. In July, Mr. Trump issued a memo taking a new approach. “For the purpose of the reapportionment of representatives following the 2020 census,” the memo said, “it is the policy of the United States to exclude from the apportionment base aliens who are not in a lawful immigration status.”
“Current estimates suggest that one state is home to more than 2.2 million illegal aliens, constituting more than 6 percent of the state’s entire population,” the memo said, apparently referring to California. “Including these illegal aliens in the population of the state for the purpose of apportionment could result in the allocation of two or three more congressional seats than would otherwise be allocated.”
In the memo, Mr. Trump ordered Wilbur Ross, the secretary of commerce, to provide him with two sets of numbers, one including unauthorized immigrants and the other without them. But how Mr. Ross would derive the second set of numbers became unclear after the Supreme Court last year rejected his efforts to add a question on citizenship to the census.