Do I have to respond to the American Community Survey (ACS)?[0]
Yes. You are legally obligated to answer all the questions, as accurately as you can.
The relevant laws are Title 18 U.S.C Section 3571[1] and Section 3559,[2] which amends Title 13 U.S.C. Section 221.[3]
As I read that, it's an infraction, with a fine of not more than $5,000 and/or imprisonment of five days or less.
I imagine such sanction would be easy enough to evade by questioning the jurisdiction, if you do not live in Washington, D.C., Puerto Rico, or on a military base, or work as a federal employee.
But in order to question jurisdiction, you will likely have to do so pro se, without an attorney (who is an officer of the court). And U.S. Attorneys can be tenacious, if needed "pour encourager les autres", so it's usually a better idea all around to just avoid their attention altogether.
The only question you really need to answer is the number of people domiciled at your mailing address on Census Day. I got the long form in 2010, and answered "four people live at this address" with no subsequent in-person harassment or prosecution.
The person who eventually arrived at my door was unaware that census data were used to place Japanese-Americans into concentration camps during World War 2, and subsequently declined to ask me my race again, or any other questions. I didn't even get to bring up post-9/11 legislation.
It turns out that the arguments I had planned on making back then were well-founded. I won't let it go to my head, on the off chance that I was checking a stopped clock at just the right time of day.
If the temp employee persists, it might be useful to ask how many people have actually been fined, since 1970, under the laws they threaten you with. You should already know the answer, so that your questions will be rhetorical. It's zero. In the last five censuses, no one has been fined for failure to respond, even though 2-3% are prime candidates.
And very much in practice. Census reps -- all those temp workers that provide a reliable employment spike -- are instructed to pay multiple home visits to anyone who hasn't completed a questionnaire, extended survey or not. They are also instructed to remind holdouts of criminal liability for not responding. And they do!
Also census reps must take an oath to not disclose individual information:
In theory.