I want to know if I am engaged in a case of Constructive Discharge

I was hired as a Maintenance Manager. Two years ago I was told by my manager that they had hired a new Maintenance Manager. It turns out this is a friend of my manager and they had been trying to get him since before my hiring date. I was told I would keep my pay and would work with the new manager. The new manager quit after 4 days. I was then reinstated as Maintenance Manager. Two years later (just a few weeks ago) I was told by the new Vice President (my former manager was fired) they had hired a new Maintenance Manager. I was again told they wanted me to stay and I would keep my pay. After 10 weeks of waiting I had to request a title or job description. I was then told that I was being demoted to Maintenance Technician (the lowest level in Maint) and would have to take a 30% pay cut or accept 4 weeks severance. The reason they gave was that there was a "perceived lack of delegation and employee leadership." This was the first I had heard of any problem with my performance. Since the Vice President who made the decision to hire the new Maintenance Manager has since been fired, I don't know where this comment came from. I believe they are trying to make me quit. I have never had a reprimand or discipline of any kind. My last evaluation was very good and I was given a large raise. Is this a case of Constructive Discharge? Do I have to give some kind of notice to my employer that I feel this is a case of CD and my reasoning why?