Like my proposal of allowing AMT-exempted tax deductible donations to departments of the federal government, the implementation would be a little tricky but the outcome incredible. There are well defined facts, questionable facts, and then “facts” that have some kind of spin. It would be difficult to stick to the well-documented facts (is some research paper published by a group who got funded by someone benefiting from the result of the research still a “fact”?). I think I would stick to opponent flagged comments too — having a limited number of wrong challenges discourages challenging every statement. But as long as your challenge is substantiated, it isn’t like you’ll find yourself halfway through the fourth quarter, fourth down two yards from the goal line, and unable to challenge a call.
Alternately candidates could be provided a list of their top n lies and told that any of these statements will be immediately challenged by the moderator. The FBI says you were careless … you can say you thought you were doing everything you could, but were found to have been careless. There are recordings of you supporting a war, you can say you changed your mind as new information came to light (why no one does this is beyond me – the “he was for it before he was against it” thing a few elections cycles ago seemed to have such an easy answer to me. I was not privy to all of the information the President had available. Based on the information we were provided at the time, I was for it. Now that new information has been made pubic, I have changed my mind) but you cannot just say you opposed it.