The census is key to establishing congressional districts and to apportioning federal funding. The Trump administration had hoped that by adding a question about citizenship amid its racist war on migrants, it could intimidate both documented and undocumented migrants into hiding from census officials, resulting in undercounting of non-native-born and racialized US residents.
The citizenship question was being proposed by running joke/Secretary of Commerce Wilbur Ross (previously), whose shitty excuses for the move were dismissed by Chief Justice Roberts as "contrived." Then Republican strategist/archvillain Thomas Hofeller (architect of the citizenship question) died, and his computer's hard drive yielded a study of Hofeller's that concluded that "a citizenship question could increase Republican political power by excluding noncitizens and underage American citizens from the census data."
It's another great example of the administrative incompetence and emotional incontinence of Trump and Co, who are so convinced that they are the smartest guys in the room (believing as they do that markets reward intelligence, rather than sociopathy) that they routinely put their secret masterplans in writing (or publish them on Twitter), so that whenever they're hauled before a judge to mumble half-assed excuses about why they're doing something that is so evil you can see it glowing from orbit, the judge inevitably is presented with the Trump regime's own explicit admission about its true motives.
The Commerce Department's claim was undermined last month when plaintiffs discovered files on the computer of a deceased Republican political strategist, Thomas Hofeller, who had urged the Trump administration to add a citizenship question to the census. Among the files was a study in which Hofeller wrote that a citizenship question could increase Republican political power by excluding noncitizens and underage American citizens from the census data used for to redraw political boundaries every 10 years.
2020 Census Form Will Not Include Citizenship Question, Trump Administration Says [Robert Mackey/The Intercept]