What's Robert Novak afraid of?

Robert Novak walked off the set of CNN's "Inside Politics" just as Ed Henry was about to ask him about his role in the outing of a covert CIA agent, Valerie Plame, in an attempt to discredit the agent's husband, Joseph Wilson.

Novak knew that he was going to be asked about this, and he made it seem that he was simply in a fit of pique over some comments from James Carville. Now, I would think that it would be easy to get incensed at Carville, but Novak's been doing this stuff too long for that to be plausible. It seems likely that he just wanted to avoid the questions about the Plame/Wilson affair. The question is, why? What's enough to make Robert Novak drop his professionalism?

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