They can and will talk about him in the debate. He had the opportunity to be there. But, expect Donald to turn the table on them and severely mock them for their attempted criticisms.
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I don’t think they will be able to talk about Trump...the candidates can only respond to the other candidates when they are mentioned by the other candidate. At least that is my understanding.
... by that logic, they can mention Trump all they want. He just won’t be there to respond.