Olds, no point to sending it in. If you haven't sent it, you're already on someone's visit list.
All the info they ask for is available through the tax assessor's office; however, by law, the Census bureau is a separate entity, and cannot collect information from any other agency. Conversely, they cannot give the information to any other agency, by law. Yes, I realize census info was used in WWII; that was an exceptional situation, and not the norm. Wartime changes things.
You can refuse to answer any question you wish. It will be noted on your form, and it will be reassigned to another enumerator, and you'll get a knock again.
Nothing is asked about SS#'s, accounts, etc. Simply names, sex, ages, race, and if you own/rent. What about that raises the hackles?
There won't be any prosecution; just up to a $5,000 fine.