Mr. Corallo, who served under Mr. Bush’s attorney general John D. Ashcroft, said he was “sort of shocked” by the volume of leak prosecutions under President Obama. “We would have gotten hammered for it,” he said.

The current administration attributes the volume of prosecutions to happenstance and the availability of evidence, rejecting accusations of politically motivated selective prosecution.

“The Justice Department has always taken seriously cases in which government employees and contractors entrusted with classified information are suspected of willfully disclosing such classified information to those not entitled to it,” a department official explained. “As a general matter, prosecutions of those who leaked classified information to reporters have been rare, due, in part, to the inherent challenges involved in identifying the person responsible for the illegal disclosure and in compiling the evidence necessary to prove it beyond a reasonable doubt.”

MR. ASHCROFT authorized a single subpoena for reporters’ testimony or records in his four years in office, Mr. Corallo said. He would not say so, but that subpoena was probably the one that troubled Judge Sack in 2006. The reporters lost. In a dissent, Judge Sack said he feared for the future.

“Reporters might find themselves,” he wrote, “as a matter of practical necessity, contacting sources the way I understand drug dealers to reach theirs — by use of clandestine cellphones and meeting in darkened doorways. Ordinary use of the telephone could become a threat to journalist and source alike. It is difficult to see in whose best interests such a regime would operate.”

What he imagined may now be reality. Consider the most recent prosecution, of John C. Kiriakou, a former C.I.A. agent who is said to have disclosed classified information to journalists in 2008 about the capture and interrogation of an operative of Al Qaeda.

Daniel Ellsberg, who provided the secret history of the Vietnam War known as the Pentagon Papers to The New York Times, said he was deeply troubled by the charges.