The practical effect of the ruling may be limited, as Census Bureau officials have said they may not be able to produce data on unauthorized immigrants until 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 the office may not be able to identify many unauthorized immigrants.
Removing large numbers of them from the census would most likely shift the allocation of congressional seats and federal money to older, whiter, and generally more Republican states.
By law, the Commerce Department is required to provide census information to the president before the end of the year, but he may not be able to meet this deadline. Mr Wall said the Census Bureau has good data on the tens of thousands of unauthorized immigrants detained by Immigration and Customs Enforcement, but that number is almost certainly too small to change the distribution.
It was less certain that the office could match records regarding those on removal orders, young immigrants known as Dreamers or other categories of unauthorized immigrants.
The Constitution requires congressional districts to be allocated “by counting the total number of people in each state”, using census information. In July, Mr. Trump issued a memorandum adopting a new approach. “For the purposes of reassigning representatives after the 2020 census,” the memo reads, “it is the policy of the United States to exclude from the allocation base foreigners who are not in legal immigration status. . “
“Current estimates suggest that a state is home to more than 2.2 million illegal aliens, or more than 6% of the state’s total population,” the memo said, apparently referring to California. “Inclusion of these illegal aliens in the state’s population for distribution purposes could result in the allocation of two or three more seats in Congress than would otherwise be allocated.”
In the memo, Mr. Trump ordered Wilbur Ross, the Commerce Secretary, to provide him with two sets of numbers, one including unauthorized immigrants and the other without them. But how Mr Ross would get the second set of numbers has become uncertain after the Supreme Court last year rejected his efforts to add a citizenship question to the census.