Adding the citizenship question to the Census is blatant in its political motivation. And yes, I'm with the gadfly hardliners who correctly point out the only thing strictly called for by the Constitution is a simple enumeration headcount.
But there's something more insidious about the Census that mostly passes unremarked. It's how the government codifies and imposes de-facto official racial categorisations in the United States. It gives sanction to the "safe" version of the question to ask, that then gets copied on practically every bit of both public and private paperwork in the country, and is used as the statistical standard for social science data and research. You probably fill out dozens if not hundreds of forms every year that ask it.
Historically, the Census has played a huge role in defining our notions of race in the United States. Of course the official intention is to just reflect common usage, not shape it, but it's like a subatomic particle: you can't observe it without affecting it.
For example: who counts as "white"? Are Arab-Americans "white"? The government says yes, and so will not include "Middle Eastern and North African" as a separate category. At one point, Indians (South Asian, not Native American) were instructed to pick white, then Asian, then finally got their own category. Those are just some of the examples as various groups have either sought to be counted separately or not be counted separately, whichever they think best serves their interests. (Historically it was usually fighting for the advantages of being deemed white, today it's often for the benefits of being identified as a distinct group.)
Roberts was probably more right than he realised when he observed "It is a sordid business, this divvying us up by race." And the innocuous dry and boring Census form, asking what's widely regarded as standard demographic information, is the foundation of how the government does it. Officially, we don't have government-imposed racial categories in the United States, and people can self-identify however they choose. De facto, everybody copies and uses the Census question and categories, because it's seen as safe and beyond reproach. Anybody else tries to create their own different categorisations, and suddenly they look like the weird racist drawing arbitrary and likely-malicious distinctions. But when the government does it for official purposes, somehow it's not seen that way.
And of course even minority advocacy groups don't want race eliminated from the Census... they rely on that data to advocate for their interests, and identify patterns of discrimination. So it's not like this is some ongoing white-nationalist plot. But the long-term effect really does have a big impact on solidifying and giving official sanction to our popular cultural notions of race, which is after all purely a social construct. We have a government agency that, in practice, gets to make decisions about what group does or doesn't count as a "race" and where to draw the lines between them.
I don't know if I have a solution. There are fair objections that the data is needed for legitimate anti-discriminatory purposes. Like you can't identify racist patterns of arrests and incarceration or funding or whatever else, if you don't have those categories and data to work with. But perhaps as a compromise: the government should get out of the business of defining the different options, and just make it an open-ended question for people to respond using whatever label they see fit. That would, at the very least, end the problem of the government perpetuating racial categories by giving them official definitions.
Or maybe we should just abolish the question altogether. Maybe its harm in perpetuating racial categories outweighs any benefit in combating historic patterns of racism. But whatever the solution, it's strange to me how little attention it gets. Somewhere in the Commerce Department there's a group of anonymous faceless bureaucrats who define, with little review or debate, one of the most important (and often-harmful) norms of American society. It's weird how little that gets discussed or disputed.