If you have tolerated it in the past, it can be very difficult to change tack. I may be older than some people on here (77 next month) but when I was a child you just did not badmouth your mother. If you did, father weighed in on mother's side, and you were in the doghouse. I was never physically punished (well, only once, and that was for something which I knew perfectly well was potentially dangerous, and forbidden for that reason. My father came upon me in flagrante and delivered just one whack across the backside before I realised he was there. I never did it again. Her was the most gentle of parents, and the smack was evidence of how much I had scared him for my own safety. I was well aware of that!)
Equally, my parents didn't badmouth me! Verbally chastising people is just a substitute for physical beatings. - if you were physically attacked by a son or daughter you would take steps to stop it happening again, similarly with a parent. Verbal attacks are just another version, one which will not involve the attacker in police proceedings. If defending yourself from an undeserved verbal attack causes alienation, then something is wrong with the basic relationship. A parent is not a scapegoat for their grown-up child's bad temper or tiredness, any more than it is legitimate to use a child as a scapegoat for a parent's unhappiness.