As two prominent Washington journalists struggle to avoid jail time over their refusal to disclose confidential sources, one of the biggest obstacles the reporters face is America’s fast-growing army of citizen Web loggers, or bloggers.

It’s not that the town criers of the online world are campaigning to send Judith Miller of the New York Times and Matthew Cooper of Time Magazine to prison. Rather, it’s the bloggers’ very existence that undercuts the journalists’ legal defense.

On Wednesday, lawyers for Ms. Miller and Mr. Cooper are scheduled to appear before a federal appeals court in the capital to argue that reporters should have a legal privilege not to testify about their sources under most circumstances. A federal prosecutor investigating whether the White House leaked the name of a CIA operative, Valerie Plame, has asked the pair to appear before a grand jury to answer questions. They have refused.

The crux of the reporters’ contention is that the public would be less well informed if journalists could not promise their sources confidentiality. However, the proliferation of blogs and bloggers could represent the Achilles’ heel in this approach. If Ms. Miller and Mr. Cooper are entitled to claim special treatment in the courts, so too could hundreds of thousands of Americans who use the Internet to post comments about their views on current events.

“They’ll say anybody with a modem and a computer is a ‘journalist,'” said a professor of media ethics and law at the University of Minnesota, Jane Kirtley. “No court is going to be comfortable with that sort of wholesale privilege.”

Ms. Miller’s attorney, Floyd Abrams, said he is bracing for questions from the court about the perils of granting legal protection to the burgeoning ranks of bloggers.

“There’s no doubt that’s the potentially dangerous aspect of it,” Mr. Abrams said in a telephone interview from his Manhattan office yesterday. “If everybody’s entitled to the privilege, nobody will get it.”

Mr. Abrams said he thinks many bloggers should be entitled to the same kind of protection he is seeking for his client and other traditional journalists. “I think a blogger who communicates with and tries to communicate with thousands of people is not less deserving than a journalist who may communicate with a smaller audience through a small-town newspaper,” the attorney said. “There should be protection so long as information was obtained for the purpose of dissemination to the public at large in some sort of analogous way to what ‘journalists’ do.”

However, Mr. Abrams said bloggers who confine their comments to matters of a personal nature should not enjoy any special privilege. “I don’t think one can sustain the position that everyone who has a Web site that they may put comments about their former girlfriends on is therefore a journalist,” he said.

So-called shield laws on the books in 31 states, including New York, establish an explicit legal privilege protecting journalists from having to testify or turn over their notes to authorities. Courts in New York have also found repeatedly that the state constitution allows reporters to decline to cooperate in criminal investigations and civil lawsuits under most circumstances.

While many journalism groups and newspaper editorial pages regularly campaign for such legal protections for reporters, some are rankled by them. In Texas, for example, efforts to pass a reporters’ shield law have broken down on several occasions in the past two decades after journalists could not agree among themselves about whether the measure was a good idea.

“The whole issue now is, who is a reporter?” said the president of the Texas Press Association, Wanda Cash. “I have great discomfort with that. Is Drudge a journalist? Probably. Is the disgruntled refinery worker who puts up a blog about Exxon a journalist? I don’t think so. The problem is, who decides?”

A former editor of the Dallas Morning News, Ralph Langer, said he worried that creating a shield law in Texas was perilously close to allowing licensing of journalists by the state.

“If the government sets the standards, then you almost have to have some form of licensing,” Mr. Langer said. “A shield law by definition means it shields some people and not others.”

In Texas, the split over the shield law proposals has divided broadcast and print journalists. Newspaper reporters and publishers have generally been wary of such a law, while television and radio stations have backed the concept. The reason for the split is not entirely clear, but it could have to do with the fact that broadcasters are already required to be licensed by the government.

An attorney for the Texas Broadcasters’ Association, Paul Watler, said he doesn’t see bloggers posing much of an obstacle to press shield laws. “You can look at the function journalists fulfill and distinguish it from what a blogger does,” he said.

Mr. Watler pointed out that most bloggers comment on facts reported by others. “I think there’s a distinction in how they go about gathering information,” he said.

While the new phenomenon of blogs highlights the potential pitfalls involved in creating a reporters’ privilege, courts have struggled for years with privilege claims from people who are not members of the traditional press corps.

One such episode stemmed from the highly publicized conviction and subsequent acquittal of a Rhode Island socialite, Claus von Bulow, on charges that he attempted to murder his wife, Sunny, in 1979 and 1980. During a civil lawsuit that followed, an “intimate friend” of Mr. von Bulow, Andrea Reynolds, asserted reporter’s privilege on the grounds that she was preparing to write about his case for a German magazine and for the New York Post. She also said she was preparing a book. The courts turned aside her claim, primarily because Ms. Reynolds did not produce any evidence that she had ever published a word about the case.

In a somewhat similar instance in Texas, a part-time college instructor and aspiring author, Vanessa Leggett, tried to claim reporters’ privilege to withhold information she gathered while preparing a book about a Houston man, Robert Angleton, who was acquitted of murdering his wife in 1997. In 2001, Ms. Leggett was jailed for nearly six months for refusing to cooperate with a federal grand jury investigating the case. The 5th Circuit Court of Appeals court found that no privilege exists for journalists facing a grand jury subpoena. It did not squarely confront the issue of whether or not to consider Ms. Leggett a reporter.

It is widely expected that the current dispute, involving Ms. Miller and Mr. Cooper, could end up before the Supreme Court. Mr. Abrams said that given the sentiments of some justices, traditional journalists could actually benefit from being lumped in with the bloggers.

“For some courts and some members of the public, the image of journalist as romantic hero had faded, but the notion of bloggers on duty to catch Dan Rather has not,” Mr. Abrams said. He said he may argue that turning aside the privilege would actually be as much as a blow to bloggers as to mainstream reporters.

“A number of members of the Supreme Court who are very hostile to the notion of special press privileges might at least take a second look at the issue. We’re not talking about a pressonly privilege,” Mr. Abrams said.