Until fairly recently (in historical terms) civil servants were not allowed to form unions in the US. Nowadays the civil service is a stronghold of unionisation in the US.
See https://en.wikipedia.org/wiki/Public-sector_trade_unions_in_... for background.
> Moe points out that Roosevelt, "an ardent supporter of collective bargaining in the private sector, was opposed to it in the public sector."[11] Roosevelt in 1937 told the nation what the position of his government was:
>> All Government employees should realize that the process of collective bargaining, as usually understood, cannot be transplanted into the public service. It has its distinct and insurmountable limitations when applied to public personnel management. The very nature and purposes of government make it impossible for administrative officials to represent fully or to bind the employer in mutual discussions with government employee organizations. The employer is the whole people, who speak by means of laws enacted by their representatives in Congress. Accordingly, administrative officials and employees alike are governed and guided, and in many instances restricted, by laws which establish policies, procedures, or rules in personnel matters. Particularly, I want to emphasize my conviction that militant tactics have no place in the functions of any organization of government employees. Upon employees in the Federal service rests the obligation to serve the whole people, whose interests and welfare require orderliness and continuity in the conduct of government activities. This obligation is paramount. Since their own services have to do with the functioning of the Government, a strike of public employees manifests nothing less than an intent on their part to prevent or obstruct the operations of Government until their demands are satisfied. Such action, looking toward the paralysis of Government by those who have sworn to support it, is unthinkable and intolerable.[12]