WASHINGTON  When a Congressional committee subpoenaed Harry S. Truman in 1953, nearly a year after he left office, he made a startling claim: Even though he was no longer president, the Constitution still empowered him to block subpoenas.

“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.

“The Bush administration overstepped in its exertion of executive privilege, and may very well try to continue to shield information from the American people after it leaves office,” said Senator Sheldon Whitehouse, Democrat of Rhode Island, who sits on two committees, Judiciary and Intelligence, that are examining aspects of Mr. Bush’s policies.