Three essential questions are involved in this situation. First, does the U.S. Constitution require that illegal immigrants be counted for purposes of apportioning House seats and electoral votes? Second, if the Constitution is ambiguous, can the Department of Commerce decide whether to include illegal immigrants? Third, do Supreme Court precedents on one-person-one-vote require that illegal residents be excluded from the apportionment count because their inclusion dilute the votes of citizens and legal residents?
The answer to the first question depends on a proper interpretation of the Constitution. The Enumeration Clause, as modified by the 14th Amendment, says representatives are apportioned among the states “according to their respective numbers,” as determined by “counting the whole number of persons in each State, excluding Indians not taxed.” The clause goes on to require a decennial enumeration—we know this as the census.
So, as a matter of constitutional interpretation, does the phrase “whole number of persons in each State” include illegal immigrants? Without getting too into the weeds, the best conclusion is that it does not. In 1789 and 1868, the nation’s borders were open, and citizenship easily obtained, so the category “illegal alien” did not exist.