*** As has been noted, the issue of class position is more complex in large and highly stratified public and private enterprises where the line between the more highly educated and well paid employees and management is often blurred. Here it’s important to note that unions have always taken the position that wage and salary earners - including many with supervisory responsibilities at all levels - should have the right to organize and bargain collectively so long as they don’t have effective control over the strategic direction of the organization. Employers have always taken the opposite view, and have sought to exclude large numbers from unions precisely on grounds they are “managers” rather than employees.
I forget the exact year (1951? Earlier? Later?) I think it could be argued that the retreat of the "left" began when the UAW refused to support the attempt of foremen to organize.