The conspiracy theory to which I subscribe goes something like this: Democrats fear that illegal aliens' votes will be exposed.
But more reasoned arguments are made by in a WSJ article the other day titled The Census Should Ask About Citizenship. It's behind a paywall, but it's imaged on the right.
There Edward Blum sets the two sides' rationale. To wit:
The worry is a political one - that the question would deter illegal aliens from returning their census forms, leading to under counts in states like California and New York which have large numbers of illegal residents. That in turn could reduce those states' apportionment of House seats and electoral votes.
Another concern is that new citizenship data could affect state legislative redistricting. There is speculation that some red states will draw districts equalized on their citizen population, rather than their total population, disadvantaging Democrats.
Mr. Blum adds this:
In a fact sheet about the ACS, the Census Bureau says it is: "We ask about place of birth, citizenship, and year of entry to provide statistics about citizens and the foreign-born population. These statistics are essential for agencies and policy makers setting and evaluating immigration policies and laws, understanding how different immigrant groups are assimilated and monitoring against discrimination. These statistics are also used to tailor services to accommodate cultural differences."
Unfortunately, expecting immigrants to assimilate these days is a form of racism to progressives dependent on the politics of identity.
For a reaction slanted toward the Democrat position, see NPR: Adding Citizenship Question Risks 'Bad Count' For 2020 Census, Experts Warn. For a defense of the question, see USA Today: Hostility to Census question is overblown.
The Supreme Court has not yet opined on the issue.
12:35 PM 1/18/2018