It's come to this

Democrats are now suing over the census.  The Hill: "Blue states sue Trump over census citizenship question."

Democratic attorneys general in several states said Tuesday they would bring legal action to stop the Trump administration from adding a question on citizenship to the next U.S. census, a question they said would lead to serious undercounts that could reverberate for years to come.

This was a question that appeared on the census until 1950, so its legality is not really in dispute.  Administration of the census is entirely the responsibility of the Executive Branch, administered through the Commerce Department.  So basically the Democrats' Constitutional objection to the new question is the legal argument of "That's no fair!"

Just don't answer the question.  It's not a deposition.