The U.S. Census always has been a vehicle for doing one thing: counting people. It is not for gathering information on individuals.
Now comes a promise from the Trump administration to ask census respondents if they are U.S. citizens. The straw-man argument for asking is to get a more accurate count of how many people can vote. Voter fraud by immigrants is a frequent refrain of the anti-immigration crowd, but it’s one for which there is scant evidence.
The more likely motivation is simply to keep immigrants from answering the door, responding to mailers or otherwise letting any census worker learn anything about them, lest it make its way to someone who can cause trouble.