Here is the proof... From the Teamsters to all CHQ pilots.

"To those pilots who are involved in a decertification effort or who may be inclined to do so, it is incumbent upon the Union to educate you on the individual consequences of your actions. Section 20.D. of the Local 747 Bylaws spells out the responsibility of a member to the Local Union as a condition of membership. This Section contains subsection 4. which states, "[n]o member shall engage in dual unionism or espouse dual unionism or disaffiliation, or be a party to any activity to secure the disestablishment of the Local Union as the collective bargaining agent for any employee." Any Union member who violates this provision can be subject to expulsion and fines in conformity with Section 21: "Charges and Trials." We realize that many of you have never been in a Union and may not be aware of the consequences of fostering cessation from the Union. This is the most serious Union crime that a member can commit."