No? You can’t forbid people from forming a union, but you can forbid things like arguing about the president.

Discussing things you may want to unionize over are often connected to the larger political realm, and cannot be severed.

Here’s an example: should a group of unionizing employees be able to say “vote for candidate foo, because they support legislation that aligns with our unions goals?”

Courts have said yes, and I agree.

The President, the Congress, the Senate, every political instance has a direct link to labor code for example. They can make or remove laws that have a direct impact on working conditions. Unions must be able to talk about all this.