In response to your inquiry about recourse options when a supervisor tells employees that going to the union is hazardous to their career advancement, the following is provided.

It is an Unfair Labor Practice for an Agency to interfere with, restrain, or coerce any employee in the exercise by the employee of any right under Chapter 71 of Title 5 of the U.S. Code Sub Chapter II 7116 (a) (1)

It is also an Unfair Labor Practice under 7116 (a) (2) to encourage or discourage membership in any labor organization by discrimination in connection with hiring, tenure, promotion, or other conditions of employment.

And  it is and Unfair Labor Practice under 7116 (a) (4)  to discipline or otherwise discriminate against an employee because the employee has filed a complaint, affidavit, or petition, or has given any information or testimony under this chapter.

So in a nutshell the supervisor who tells employees that going to the union will kill their career advancement is committing multiple violations of Federal law. 

Moreover, the Master Labor Agreement, at Art. 3 Sec. 3.01 establishes that officials of the Employer  are governed by all applicable laws, rules, and regulations. And, Sec, 3.04 contractually grants each bargaining unit employee the right to form, join, or assist any labor organization, or to refrain from any such activity, freely and without fear of penalty or reprisal, and each employee shall be protected in the exercise of such right.

So in another nutshell, a supervisor who tells employees that going to the union will kill their career advancement is also violating the collective bargaining agreement between AFMC and AFGE Council 214.

So you would have 2 options to pursue recourse. An Unfair Labor Practice Charge could be filed through your local union or you could file a contractual grievance, but you can’t do both. I would advise you to contact your local for assistance in correcting this serious violation of your rights to participate in protected activity.