There is probably a better source, but general examples of what I mean can be found here. [0] More details regarding discrimination based on union sympathies can be found here. [1]

[0] https://www.eeoc.gov/youth/what-employment-discrimination [1] https://www.nlrb.gov/about-nlrb/rights-we-protect/the-law/di...

I support the right of private employers to discriminate against employees for anything, including despicable premises like sex and race. If you are the government, you should only be allowed to discriminate based on actions that could adversely affect government efficiency, and being party to a union falls under that.