Unless they are trying to flip Libby and have him say that Cheney directed him to contact various reporters off the record and mention that Wilson's wife worked for the Agency. Cheney would be questioned directly on that point as well to determine whether this was part of a concerted strategy to discredit Wilson. This is what the Dems are hoping for.
But even if that is the motive...a videotaped testimony is what Clinton got...so why is Cheney treated differently? I think we ALL know