I’ve read Trump’s July 21, 2020 memo (www.whitehouse.gov/...); I am a lawyer; and that memo will not stand up in court.
The 14th Amendment (Section 2) is very clear that “Representatives shall be apportioned among the several States according to their respective numbers, counting the whole number of persons in each State, excluding Indians not taxed.” Non-citizens, including undocumented immigrants, are “persons” who must be counted and included in the apportionment, if they are “in” a state. Trump’s memo accepts this textually plain premise. But it claims that “in” really means “inhabitant of”:
Although the Constitution requires the “persons in each State, excluding Indians not taxed,” to be enumerated in the census, that requirement has never been understood to include in the apportionment base every individual physically present within a State’s boundaries at the time of the census. Instead, the term “persons in each State” has been interpreted to mean that only the “inhabitants” of each State should be included. Determining which persons should be considered “inhabitants” for the purpose of apportionment requires the exercise of judgment.
So far, fair enough. See Franklin v. Massachusetts, 505 U.S. 788 (1992), which the memorandum cites (for the proposition that the President has some discretion in specifying the details of the apportionment calculation). But Franklin also holds (9-0 in this respect) that the meaning of “in” is informed by the practical construction adopted by the first Congress in passing the first enumeration Act, under which persons were counted as being “in” the State of their ”usual residence," meaning they have “some element of allegiance or enduring tie to” that place. Id. at 804. The thing is, undocumented immigrants and other non-citizens are also “inhabitants” of the State where they reside, regardless of their citizenship and immigration status, under the same “enduring tie” test that is used to assign travelling citizens and the like (in Franklin, service members posted abroad) to a state of “usual residence.”
Under the textualist approach used by Justice Gorsuch for the majority in Bostock v. Clayton County (last month’s decision on sexual orientation workplace discrimination), the analysis can probably stop there. But in addition, note that the traditional construction under which “in” = “inhabitants” = “usual residents” was also known to the authors and ratifiers of the 14th Amendment. Thus, when they required apportionment based on all “persons” “in” a state (rather than distinguishing, as had Article 1, Section 2, Clause 3, between “free persons” who were counted one-for-one, and “all other Persons,” who were counted as three-fifths), they adopted the “usual residents” construction. They could have, but did not, distinguish between citizens and other inhabitants. They may have been tempted to do so, in order to punish, with a loss of apportionment, former Confederate States that did not treat former slaves as citizens. But they chose instead to treat all persons usually residing in each state as equal for census and apportionment purposes.