Bloomberg in turn accused the union of walking away from negotiations over a new teacher evaluation system.
The UFT ad released Friday accuses Bloomberg of "going after teachers" instead of supporting them.
Bloomberg responded in his radio show on WOR. He said calling people "bad things" in ads makes it unlikely that the parties will reach an agreement.
The city and the union have been unable to agree on a new system for evaluating teachers. The city risks losing $250 million in state aid if there is no deal by Jan. 17.
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