It was a facetious argument if you ask me. If the "executive" branch can't investigate Congressmen, then there's really no one else with the constitutional authority. Granted, they could form their little daisy chain subcommittees and smooth it all over for each other, but saying the justice department can't touch congressmen in their offices is giving them an get-out-of-jail free card. The last thing we need is some kind of fekked up diplomatic immunity for our OWN congressmen...