“If the doctrine of separation of powers and the independence of the presidency is to have any validity at all, it must be equally applicable to a president after his term of office has expired,” Truman wrote to the committee.

Congress backed down, establishing a precedent suggesting that former presidents wield lingering powers to keep matters from their administration secret. Now, as Congressional Democrats prepare to move forward with investigations of the Bush administration, they wonder whether that claim may be invoked again.