Don't quit, let them fire you and seek an attorney. Write EVERYTHING down and keep a log and timeline. Don't give an inch. It will only benefit you and your attorney. Any e-mails, notes, anything, keep and add to the file. Whether you get unemployment or not, you may be able to win a case of harrassment. Don't let them scare you into giving up on getting anywhere.
Don't follow my advice because it is probably wrong, but I would not resign. I would force them to fire me and then consider my options. I would ask my union rep for advice and might even seek legal advice. Good luck with this situation and your recovery.
