I'll just post what is on the card in BOLD print...
"Now is the time to complete this survey if you have not already done so"
Your response to this survey is required by U.S. law, if you do not respond, a Census Bureau interviewer may contact you to complete the survey.
There problem as far as I'm concerned is the survey was not addressed to me personally but addressed to RESIDENT, and everything I get addressed resident goes straight in the trash as with all junk mail.
If they do show up at my door, and I have the time, I will answer only those questions I think do not violate my right to privacy, or I'll just tell them to go away.
When anyone says the US law requires blah, blah, blah, without citing any actual law, they are usually just making it up or exaggerating big time.