The Obama Justice Department's crusade against leakers just took a quantum leap - and it's extremely worrisome.

It's one thing to go after officials who leak classified information to the press. The Obama administration has gone after more of them than all previous administrations combined. Nonetheless, officials with security clearances sign a contract pledging not to share material with the outside world - and they know they could face criminal penalties if they do. (Daniel Ellsberg figured he might go to prison for leaking the Pentagon Papers and was willing to make the sacrifice.)

However, it's something else entirely to go after a reporter who receives the leak. That's what federal prosecutors are doing to James Rosen, the chief Washington correspondent for Fox News. And they're going after him not as a witness to a crime - nor as a pressure tactic to get him to give up his source (in this case, the source has already been caught) - but rather, in the words of a Justice Department affidavit, as "an aider, an abettor, and/or a co-conspirator": in short, as someone who might be indicted under the Espionage Act.

This has never happened in this country. (Even in the Pentagon Papers case, several newspapers were served injunctions not to publish stories, but no reporter or editor was ever investigated, much less tried, as a co-conspirator.) If the prosecutors go through with their threat, the entire enterprise of national security journalism - which inherently involves uncovering secrets, to some degree - will be in jeopardy.

A similar case occurred in 2006, during George W. Bush's presidency. In United States v. Lawrence Anthony Franklin, et al., prosecutors indicted a Pentagon official and two policy analysts with AIPAC under the federal espionage statute. The official, Larry Franklin, was charged with leaking classified information. (He pleaded guilty and served a brief sentence.) The two AIPAC analysts, Steven Rosen and Keith Weissman, were charged simply with receiving it.

The section of the indictment titled "Ways and Means of the Conspiracy" found that Rosen and Weissman:

". . . would cultivate relations with Franklin and others and would use their contacts within the U.S. government and elsewhere to gather sensitive information, including classified information, relating to national defense, for subsequent unlawful communication, delivery and transmission to persons not entitled to receive it."

Those final words are worth noting. They were charged with giving classified information not to foreign governments or spies but simply "to persons not entitled to receive it."

As I noted in a Slate column at the time, "This is what journalists do routinely every day. They receive information from insiders, write it up in a story, send it to editors, who publish it in newspapers, magazines, wire services, or on Web sites . . . which are seen by readers who have not been officially authorized to view that classified material."

If Rosen and Weissman were prosecuted for joining in a "conspiracy" with Franklin on the grounds that they'd received classified material, then The New York Times, Washington Post, The New Yorker or any number of other publications could be next.

The presiding U.S. District Court judge, T.S. Ellis, made this same inference, noting that the law "applies to academics, lawyers, journalists, professors, whatever." In the end, though, Ellis dismissed the indictment against the AIPAC analysts for precisely that reason, noting that the statute was too vague, broad and draconian.

Now, though, President Obama's prosecutors are pushing the logic of the AIPAC case to its absurd extreme: They're labeling a reporter who received classified information as a criminal abettor or co-conspirator.

Rosen's Fox News story, which appeared in June 2009, did contain serious, possibly damaging intelligence information. It reported that North Korea was likely to respond to some United Nations sanctions, which had just been handed down, by conducting a nuclear-weapons test. It also reported that the CIA learned of this from sources inside North Korea.

The key fact, from a prosecutor's viewpoint, was that any North Korean official reading this story would now know that the CIA had sources inside North Korea - and that the source (whether it was a human spy, a phone tap or whatever) would subsequently be shut down, one way or another.

Rosen's source for the story was Stephen Jin-Woo Kim, a State Department adviser and one of a small number of officials who received a top-secret memo about the CIA finding. Few dispute the Justice Department's right to prosecute Kim, who did break his security pledge (though many are critical of how often, and how relentlessly, the Obama administration pursues leakers). One might also criticize Rosen or his editors for not writing the story in a way that finessed the fact that the information came from a source inside North Korea. But that's different from arresting him under the Espionage Act.

It's possible that the prosecutors won't indict Rosen, that they called him a criminal co-conspirator in their court affidavit to boost the chances that the judge would approve their request to track his email, phone calls, and movements. However, even if that is the case, it's a dangerous game. Some future prosecutor could use the Rosen affidavit as a precedent to go all the way, to treat some similar reporter - potentially all reporters on the national security beat - as criminal conspirators.

Fred Kaplan is Slate's "War Stories" columnist and author of the book "The Insurgents: David Petraeus and the Plot to Change the American Way of War."