I'd like to know the legal history of labor unions in the U.S.A. Has the constitutionality of labor unions ever been challenged? If so, which amendment or amendments allow their existence? How were the first labor unions established and recognized under law?
There's no constitutional issue with labor unions. Unions are simply an organization of people joining towards a common goal. What could be constitutionally challenged are the laws which allow unions to exist and provide certain protections for their members.