Adding the citizenship question helps red states
Deleting the citizenship question helps blue states
It's all about political power, so what else is new...
Actually, what it does is makes the census less accurate.
Obviously, none of you nitwits has ever worked as a census enumerator.
We don't count by people, we count by residences.
So here's what happens. They send out the Census forms, and about 70% of people actually bother to fill them out and return them. (This is what happened in 2010). The Census bureau then hires a bunch of "Enumerators" to go to each and every address that did not return a form. (I've done this twice in 2000 and 2010).
Most people are cooperative, but you get a few who aren't. Some are indeed undocumented citizens. Some of htem are right wing nuts who think that the government merely knowing where you live is part of some vast globalist conspiracy to put a micro-chip in their ass. If we can't get the person to come to the door, we ask their neighbors.
So the enumerators gather as much info as they can, and then the rest are passed along to the next level of checking, which usually involves checking public records, school records, utility companies, and so on.
Now, here's the thing. Undocumented immigrants are already under-counted, because they know how to stay under the radar. So Trump's usual modus-oporendi of just being a mean prick really won't have that much of an effect. The Census Bureau will figure out someone lives there with three kids, and that will still count, their citizenship status not being germain to the issue.