After you have documented a substantial pattern of abuse, made allies, collected witness statements from well-groomed supporters, and done everything in your power to disarm your bully (such as putting a bouquet of flowers on your desk the day after an incident to show him or her that you are feeling just fine, thank you), then it may be time to seek outside help.
There are agencies such as the Equal Employment Opportunity Commission, the National Labor Relations Board, the Occupational Health and Safety Administration, and the Department of Labor. All of these have information at their websites that might be relevant or useful.
There are unions, and of course, your HR department. There are also many aid and support groups that focus on representing a special issue or population, such as women or a particular minority. Among the more notable organizations are The National Organization of Women, La Raza, NAACP, and the Asian Law Caucus.
There are also attorney groups like the National Lawyers Guild that represent multiple cases, as well as the ACLU, which specifically defends our constitutional rights, such as those associated with the First Amendment. Labor attorneys and workplace conflict counselors are other good options.
However you choose to deal with your bully boss, be a workplace warrior, not a victim.