The US Constitution and the rights and protections guaranteed therein apply to all persons within the US border. The census, as it is designed, is to count all persons in the US. It does NOT say that only citizens should be counted, but all persons. By removing the ‘citizen question’ the government can be more certain to collect more census data, putting the US government closer to being able to accurately fulfill its mandate to count every person. Remove the question, fulfill the mandate.