A letter from a reader:
I've been getting a lot of questions on this subject from fellow students.
As both a Libertarian and a military veteran, I find the numerous, intrusive questions asked by the Census greatly disturbing. Are the questions, besides the ones directly related to the number of people in your household, unconstitutional? Personally, I believe that they are. If I refuse to answer questions other than those pertaining to the size of my household, what, if any, legal consequences could I face? I ask because I can't seem to find any concrete information on the subject. Also, if I may ask, how do you all plan on approaching the Census?
Dear Reader: Yes, I happen to know the answer to that.
The American Community Survey is conducted under the authority of Title 13, United States Code, Sections 141 and 193, and response is mandatory. According to Section 221, persons who do not respond shall be fined not more than $100. Title 18 U.S.C. Section 3571 and Section 3559, in effect amends Title 13 U.S.C. Section 221 by changing the fine for anyone over 18 years old who refuses or willfully neglects to complete the questionnaire or answer questions posed by census takers from a fine of not more than $100 to not more than $5,000. The U.S. Census Bureau may use this information only for statistical purposes. We can assure you that your confidentiality is protected. Title 13 requires the Census Bureau to keep all information about you and all other respondents strictly confidential. Any Census Bureau employee who violates these provisions is subject to a fine of up to $250,000 or a prison sentence of up to five years, or both.
Here is the reference for Title 13, if you want to read it
Now, you asked if the law making responses mandatory is constitutional. You could mean, "where is it in the Constitution?" The fact is that there is SOMETHING there, about the Census, but of course not the intrusive questions. But Article I, Section 2, Clause 3 does have a provision that a counting be done, "in such manner as [Congress] may direct."
Or you could mean, "What Supreme Court case contains the decision that justifies the law?" In that case, here is a legal history