Earlier quoted context omitted.
Contributions to unions aren’t capped and don’t need to come from members. If George Soros wants to leave his fortune to the AFL-CIO I doubt they’ll refuse.
Unions are directly elected institutions with general assemblies. That means you can't ever capture a union using money alone. Moreover, unions are legally prevented from contributing to political campaigns, while corporations are allowed to do so. Therefore, the comparison breaks down at every single level, and no useful conclusion can be reached from it.
Political speech rights in the US belong to individuals, who do not lose them by coming together in corporate organizations, whatever their form. This is just.