If you had a union there would be a mechanism for accepting an unsafe assignment under protest. It would be a form that you would complete stating why in your opinion the assignment was unsafe, to whom you spoke to request additional staffing, and an assertion that you will do your utmost to prevent a bad outcome but you can't be held ultimately responsible.
If you have no union and no pre-printed form, you can put this information on a blank sheet of paper. You keep a copy, management gets a copy and you send copies to whatever outside entities have an interest in the quality of care. (State nursing home licensing agency, Medicare, etc.).
But don't do this without starting a new job search
. Because this is going to enrage your management and without a union your goose will be cooked. If I was in your shoes, I'd find a new job first, then drop a dime on my way out the door. Good luck.