I am unsure when or how a connection between the two formed but both anti-labor conservatives and tankies claim that unions are a stepping stone to communism. I don’t believe that unionization and communism are mutually inclusive.
You can indeed be pro-union and anti-comunist. As a matter of fact all the EU countries have strong pro-union laws and are not communist countries nor they want to be.
As an example, in Spain (where I’m at) if a company has more than 80 employees they MUST have a union. If they don’t, the company will get a fine.
The americans way of looking at these things is so tiring. Every european reads ops question and shakes his head propably. There has to be a lot more that’s going wrong than “just” Fox News and Trump.
Communists are doing their fair share of trying to subvert the concept of unions. It doesn’t help that several unions use communist imagery.