I've looked it up: the cause of action for being forced to resign is called Constructive Discharge. You will have 180 days to file a written complaint with the EEOC. So you must act promptly. Here is from the website of a Pennsylvania labor law firm that represents employees and former employees and you should be able to find same in your state:

"Can I Sue for Being Forced to Resign?
If you were forced to quit or resign from a job due to intolerable working conditions, you may be able to sue your employer for constructive discharge. Legally, constructive discharge is a form of termination because you were forced to quit against your will. If you are forced to resign, you should be able to receive unemployment benefits. You are also able to file a complaint with the EEOC. It is important to keep a record of everything that occurred while working for your employer to support your constructive discharge claim. Our experienced employment law team can help with this process, and we recommend contacting us as soon as possible. Filing a claim with the EEOC must be done within 180 days if you work in the private sector and 45 days after termination if you work for the government. At [law firm], we have been helping clients for over 35 and we're here to answer your questions. We're available for free legal evaluations 24/7 at [phone], by chat, or by filling out the form on this page."
Is it Better to Quit or Be Fired? | Edgar Snyder.