The question of whether General Patraeus would become "General Betray Us?" was not moveon's argument. It was the question to which their argument lead them. In their opinion, General Patraeus made misleading statements in the past and had painted an overly optimistic picture of progress in Iraq. I believe that many people, including some among the Pentagon brass, agreed with much of moveon's factual premise. Based on their good faith belief that he had mislead in the past, moveon concluded that there was reason to suspect he would do so again. That's not ad hominem.

If you want to ignore the facts they stated in support of their argument, then I guess you can call it argumentum ad hominem, but I don't see the point in doing that.