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They aren't permitted to ask each other questions, propose pledges to each other, or walk outside a "predesignated area." And for the town-hall-style debate tomorrow night, the audience members posing questions aren't allowed to ask follow-ups (their mics will be cut off as soon as they get their questions out). Nor will moderator Candy Crowley.
Most bizarrely, given the way the debates have played out, the rules actually appear to forbid television coverage from showing reaction shots of the candidates: "To the best of the Commission's abilities, there will be no TV cut-aways to any candidate who is not responding to a question while another candidate is answering a question or to a candidate who is not giving a closing statement while another candidate is doing so." The "best of the Commission's abilities" must be rather feeble, seeing as how almost every moment of the two debates so far was televised in split-screen, clearly showing shots of a "candidate who is not responding to a question while another candidate is answering a question."
Which means some of the rules below that both campaigns stipulated to in a desperate attempt to wring any serendipity out of the events may be honored in the breach:
"The candidates may not ask each other direct questions during any of the four debates."
"The candidates shall not address each other with proposed pledges."
"At no time during the October 3 First Presidential debate shall either candidate move from his designated area behing the respective podium."
For the October 16 town-hall-style debate, "the moderator will not ask follow-up questions or comment on either the questions asked by the audience or the answers of the candidates during the debate...."
"The audience members shall not ask follow-up questions or otherwise participate in the extended discussion, and the audience member's microphone shall be turned off after he or she completes asking the questions."
"[T]he Commission shall take appropriate steps to cut-off the microphone of any...audience member who attempts to pose any question or statement different than that previously posed to the moderator for review."
"No candidate may reference or cite any specific individual sitting in a debate audience (other than family members) at any time during a debate."
For the town-hall debate: "Each candidate may move about in a pre-designated area, as proposed by the Commission and approved by each campaign, and may not leave that area while the debate is underway."
The 2012 Debates - Memorandum of Understanding Between the Obama and Romney Campaigns