According to the FBI:
Domestic terrorism is the unlawful use, or threatened use, of force or violence by a group or individual based and operating entirely within the United States or Puerto Rico without foreign direction committed against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof in furtherance of political or social objectives. (source)
In Michigan, union protesters physically assaulted Steven Crowder of Fox News, then destroyed a tent belonging to Americans for Prosperity (video below).
The union activity caught on tape involved 1. unlawful 2. violence and/or use of force 3. by a group 4. based in the United States 5. without foreign direction (that we know of) 6. committed against persons (Crowder) and property (AFP) 7. to intimidate or coerce the governor of Michigan (government) and members of AFP (civilian population) 8. in furtherance of political or social objectives
Therefore, unions – at least those involved in the Michigan terrorism – are terrorist organizations. International unions would seem to be international terrorist organizations.
Funding of terrorist organization is a federal crime. Logically, then, anyone who voluntarily contributes to a union willfully and unlawfully supports a terrorist organization.