Congress has long been concerned about the use of the Internet as an anonymous medium for the sale or exchange of child pornography.

In response, law-enforcement officials have maintained an aggressive posture through undercover operations to shut down a thriving illicit market that victimizes children.

But emerging technology is presenting new challenges to these enforcement efforts, particularly with the creation of "virtual" child pornography, images generated by computer.

On Tuesday, the US Supreme Court takes up a case that examines whether a 2003 federal law aimed at addressing the virtual child-pornography problem casts too wide a net that might also infringe constitutionally protected speech.

At issue in US v. Michael Williams is whether an individual can be arrested and sent to prison for a mandatory five to 20 years for attempting to arrange a swap of photographs that are believed to be illegal child pornography.

Under a 2003 child-pornography law, Congress empowered federal agents to arrest anyone for advertising, promoting, presenting, distributing, or soliciting material in a manner that is "intended to cause another to believe" that the material is illegal child pornography. The law applies even if the underlying material isn't actually child pornography.

US Solicitor General Paul Clement says the law, a section of the Protect Act, is a carefully calibrated effort by Congress to safeguard children from sexual exploitation by targeting those who would traffic in child pornography.

Critics say the law is a vague and overbroad regulation of free speech that threatens to establish the federal government as a kind of thought police. They say the government should punish illegal conduct, not controversial – or even repulsive – thoughts and fantasies. In addition, they say the law could chill artistic, literary, scientific, and other forms of protected speech.

"How are you as a promoter or as a describer supposed to know what is likely to make another person 'believe?' " asks John Feldmeier, a political science professor at Wright State University in Dayton, Ohio., who helped file a friend-of-the-court brief on behalf of the Free Speech Coalition, a trade association for the adult-entertainment industry.

The issue arises in the case of Michael Williams of Key Largo, Fla., who was arrested in May 2004 after an encounter with an undercover agent in an adult chat room on the Internet.

Mr. Williams logged into the chat room and posted a message that he had "good" photographs of his 2-year-old daughter that he wished to swap for similar photos. The undercover agent responded and engaged Williams in a private Web chat. The agent identified himself as a 30-year-old mother with a 10-year-old daughter.

Williams and the agent swapped photos. The children depicted in the photos were clothed and were not engaged in sexually explicit activities.

According to court documents, Williams tried to get the agent to provide a more sexually explicit photograph. When it did not arrive, Williams posted a warning message in the chat room that the undercover agent was a cop. The undercover agent responded in a chat-room message accusing Williams of being a cop.

In response to the accusation that he was an undercover cop, Williams posted a hyperlink to seven sexually explicit photographs of children from ages 5 to 15.

Four days later, federal agents executed a search warrant for Williams's trailer in Key Largo. They discovered 22 computer images of children engaged in various forms of sexual activity. They also discovered that Williams lived alone and did not have a 2-year-old daughter.

Williams was charged with possession of child pornography. But federal prosecutors did not stop there. They also charged him with violating the federal child-pornography pandering law for his Internet encounter with the agent.

Williams agreed to plead guilty to both charges, but reserved the right to appeal the pandering conviction. His lawyers say he shouldn't be held criminally liable for false claims expressed in an adult chat room.

The 11th US Circuit Court of Appeals in Atlanta agreed with Williams and reversed the pandering conviction.

In appealing to the Supreme Court, the Bush administration argues that the law is aimed at the kind of pandering that sustains the illegal trade in child pornography – which in turn poses a threat to the well-being of children.

Part of the reason Congress passed the pandering statute was to give federal authorities the ability to prosecute individuals selling or trading computer-generated child pornography.

In 2002, the Supreme Court struck down an attempt by Congress to extend a ban on child pornography to include computer-generated child pornography. The ruling opened the door to distribution of virtual child porn. It also created a potential defense for individuals arrested for possessing actual child pornography. They could claim that the children depicted weren't real children.

Even if they were real, the virtual pornography claim would complicate any prosecution by forcing the government to prove the children in any photos were real.

Congress responded by enacting the 2003 law allowing federal agents to arrest someone for presenting or soliciting virtual child pornography if the would-be recipient believes it is child pornography involving real children.

In the Williams case, it enabled the government to prosecute Williams for trying to distribute child porn that never existed involving a 2-year-old girl who also doesn't exist.

Lawyers for Williams say the law's reliance on "beliefs" gives too much discretion to investigating agents. "The speaker's criminality does not depend upon his or her intent, but rather on what the speaker's audience believes the speaker is talking about, even if that belief is deluded," says Williams's attorney Richard Diaz of Coral Gables, Fla., in his brief to the court.

"It enters into an area that should be 100 percent off-limits to any form of government regulation," Professor Feldmeier says. "Government should not be regulating the mind."

Government lawyers have a different perspective. "The statute does not punish mere 'thoughts,' but applies only when an objectively reasonable person would conclude from the context that the speaker is offering or seeking real child pornography," Solicitor General Clement writes in his brief.