Employers may ban union representatives from promoting their union in a public space within the employer’s facilities if those representatives aren’t employees, the National Labor Relations Board ruled June 14.

The decision issued by the Republican majority of the labor board overturns a near 38-year old precedent that allowed nonemployee union representatives to use public areas of an employer’s workplace, like public cafes, to solicit for or promote their union membership so long as their actions weren’t disruptive.

Public spaces...