<< Trade unions were outlawed as criminal conspiracies until the New Deal , NLRA legislation. Criminal syndicalism >>
Nah. The Criminal conspiracy view is 200 years ago, see the Philadelphia Cordwainer's case, 1806. By the 1840s, the doctrine had shifted to civil rather than criminal consipiracy. You get a stort of mid-stage view in Vegelan v. Gunter, 44 N.E. 1077 (Mass. 1896), with a nice dissent from Holmes, then on the Mass. S.J.Ct. There the court affirmed an injunction in a picket of a nonuinion employer to dissuade workers from seeking employment there. Holmes said, no, moral intimidation should be OK, although not physical intimidation, after all, we are talking class struggle here. (Yes, H was quite open about the class struggle. Courts were in those days.) Anyway, you had a lot of labor injunctions against strikes and picketing and organizing under 1932, with the Norris-LaGuardia or anti-injunction Act.
For cases and discussion, see Archibald Cox, Labor Law, 12th ed.
--jks