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This all depends on your contract. If your contract provides for the overtime list, your supervisor cannot force you to work overtime contrary to the contract. this is grounds for a grievance with the union under the contract and grieve this. If the union is refusing to pursue the grievance as they are mandated to do under the contract, then this is an unfair labor practice and you would go to the National Labor Relations Board against the union for breach of your contract in refusing to properly represent you as required as part of your contract with them to represent you.
That is your only remaining recourse if the the union is not filing a grievance as they should.