https://www.nlrb.gov/rights-we-protect/whats-law/employers/discriminating-against-employees-because-their-union
Discriminating against employees because of their union activities or sympathies (Section 8(a)(3))
It is unlawful to discourage (or encourage) union activities or sympathies "by discrimination in regard to hire or tenure of employment or any term or condition of employment." For example, employers may not discharge, lay off, or discipline employees, or refuse to hire job applicants, because they are pro-union.