The Trump administration wants to deduct from census counts unauthorized persons when Congressional seats are apportioned. I have already posted on this. The Migration Policy Institute describes how it believes the administration plans to do it, even though it does not ask in the census a question about citizen status, much less unauthorized status. It plans do it by matching records of disparate federal databases.
Some anti-immigration groups have tried matching, name by name, between voter records and driver registrations, a conceptually simpler task, and end up with unusable messes. (See here for misadventures in TX, FL, SC and NH>)
An Executive Order of July 11, 2019 orders Executive Branch agencies to cooperate with the Census Bureau by using “administrative records” “the number of citizens, non‑citizens, and illegal aliens in the country.” The MPI says this means matching Census records with social security, Medicare, Medicaid, IRS, Homeland Security records perhaps state records. “The problem is that millions of citizens and legal immigrants cannot be matched to administrative records.”
The MPI says that prior efforts to match records “suggests up to 20 million U.S. citizens could be excluded” from the count of citizens.”
A 2018 internal analysis by the Census Bureau on such matching concluded it would not work as desired. The main problem is that surveys, such as the Census, produce “significantly lower estimates of the noncitizen share of the population than would be produced from currently available administrative records.” The authors cite “noncitizen respondents misreporting their own citizenship status and failing to report that of other household members. At the same time, currently available administrative records may miss some naturalizations and capture others with a delay.” The MPI adds that some “may use a different version of their name on the Census form than used in government records.”
The Census undercount will be worse “in densely populated urban areas where more people crowd in each housing unit, and in rural agricultural areas where migrant farmworkers live for part of the year. It is also difficult to collect data from younger people, especially those who move back and forth from home during their college years.”