Wis. Stat. § 103.54: Responsibility for unlawful acts.

No officer or member of any association or organization, and no association or organization participating or interested in a labor dispute may be held responsible or liable in any civil action at law or suit in equity, or in any criminal prosecution, for the unlawful acts of individual officers, members, or agents, except upon proof by a preponderance of the evidence and without the aid of any presumptions of law or fact of all of the following:

(1) That individual officers, members or agents of the association or organization committed those acts.

(2) That the officer or member, or association or organization, sought to be held liable or responsible actually participated in or authorized those acts or ratified those acts with actual knowledge of those acts.