Employment Law Questions? Ask an Employment Lawyer.
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Unfortunately, failure to disclose can be grounds for them to refuse to hire you, but also the conviction itself is a basis for them to deny your employment. The union can seek to file a grievance and go to arbitration and if they did so and were not successful then your SOLE recourse and option is filing suit in court by proving that their termination violated your contract with them. If you cannot prove the violation of the contract with them, then I am afraid the court cannot help either.