If you can't tell that people are illegal, they obviously must be counted as citizens for apportionment of representatives. That is the whole purpose of the Census.
Dilute citizens with illegals when carving out districts, and dilute citizens with illegals' votes on every election day.
Suppose for example, there is a mythical county called Sagittarius County, in the state of Maximus. Now theoretically, the Government already knows the approximate number of U.S. citizens in Sagittarius county because of the all the records already in the possession of all Government agencies.
So, Sagittarius County, in Maximus State, has a certain number of House members based on its population, which it reported as being 800,000 in the 2010 census. After the paperwork is in for the 2020 census, the Government determines that there should only be approximately 500,000 U.S. citizens in Sagittarius County, and the rest must be here illegally. Thus, the number of members in the House of representatives in the State of Maximus is proportionally reduced! And, Government funding based on what the true population should be is ALSO reduced!
If this is not what is being considered, then why did President Trump drop the question from the Census?
IOW, illegal representation.