When you are a sitting Senator and you don't have disclose your mortgage as a liability on your financial and ethics disclosure report.
Given that a strong minority of Senators are actually millionaire's maybe they don't carry a mortgage. But in reality I doubt that since the mortgage interest deduction is the one solid deduction you are guaranteed to have. So why shouldn't it be disclosed, particularly in light of the sweetheart mortgage deals that the Subprime Six have received.
Kudos to Senator Cornyn for his move.