I don't myself have any expertise on the Census citizenship question case (which has mostly been litigated as an administrative law case), but I thought Prof. Blackman's query was intriguing, and wanted to post it:

The Fifteenth Amendment, ratified in 1870, provided that the "right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude." The Fourteenth Amendment, ratified two years earlier in 1868, provided another mechanism to prevent states from disenfranchising the freedmen: states that abridged the right to vote would lose representation in Congress.

Section 2 has three relevant clauses (emphasis added):

"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. "But when the right to vote at any election for the choice of electors for President and Vice President of the United States, Representatives in Congress, the Executive and Judicial officers of a State, or the members of the Legislature thereof, is denied to any of the male inhabitants of such State, being twenty-one years of age, and citizens of the United States, or in any way abridged, except for participation in rebellion, or other crime, "the basis of representation therein shall be reduced in the proportion which the number of such male citizens shall bear to the whole number of male citizens twenty-one years of age in such State."

First, representation is allocated based on the "whole number of persons." This provision expressly repudiated the three-fifths clause, which allocated representation based on "three fifths of all other Persons," that is, slaves. Indeed, the census is now responsible for counting the "whole number of persons," that is the "actual enumeration."

Second, the Fourteenth Amendment asks if male citizens over the age of twenty-one are denied the right to vote in state or federal positions. If so, there is a penalty.

Third, the Fourteenth Amendment mandates that offending states will lose representation in Congress in "proportion" to "the whole number of male citizens twenty-one years of age in such State."

While the census counts the "whole number of persons" to establish the "actual enumeration," Section 2 of the Fourteenth Amendment requires an additional piece of information: "the whole number of male citizens twenty-one years of age in such State." (I will table for now whether subsequent amendments—the Nineteenth Amendment and Twenty-Sixth—impliedly modified that clause to also require a count of the number of female citizens over the age of eighteen.)

If this argument is correct, then the government would not only be justified in counting the number of citizens in a given state; Section 2 would require the government to have this information available, in the event that a state deprived males (and females) of the right to vote. This is an obligation: the representation "shall be reduced." And Congress would not be able to enforce Section 2 without an additional enumeration based on citizenship. An estimate would not suffice.

Moreover, the number of citizens must be counted in advance. It would subvert the operation of Section 2 to wait until the next decennial census to count citizens. The waiting period could be as long as ten years. This information need not be collected in conjunction with the census. But it would be reasonable for the government to utilize the census process to collect this additional information. And once the government knows the total number of citizens, it can then discount minor citizens.

Professor Kurt Lash, who compiled an exhaustive two-volume set about the Reconstruction Amendments, tells me that certain documents in the historical record seem to anticipate using the census to enforce section 2. See The Reconstruction Amendments: Essential Documents" (2 vols) (Kurt T. Lash, ed.) (forthcoming U. Chicago Press). For example, in December 1865, Rep. Thaddeus Stevens discussed proposed text of what would become Section 2. He said, "A true census of the legal voters shall be taken at the same time with the regular census." In May 1866, Senator Jacob Howard gave a famous speech introducing the Fourteenth Amendment. He stated that "where a State excludes any part of its male citizens from the elective franchise, it shall lose Representatives in proportion to the number so excluded." Howard's speech makes several references to the role that the census plays in enforcing this requirement.

This research is quite cursory. I did not see this issue briefed anywhere in the census litigation. I raise the question here, and welcome any feedback.