PHILADELPHIA — It seemed like a good idea: enact a federal law to protect children from sexually explicit material on the Internet.

But eight years after Congress passed the Child Online Protection Act, legal challenges from sexual health sites, the online magazine Salon.com and other Web publishers have kept it from being enforced.

The 1998 law would impose a $50,000 fine and six-month prison term on commercial Web site operators who publish content "harmful to children," as defined by "contemporary community standards." Opponents say that definition is so broad it would stifle free speech.

Now, technology experts and others oppose the law on more practical grounds — they say it's obsolete.

Parents today are more concerned about online predators than racy pictures, said University of Pennsylvania law professor Polk Wagner, who teaches intellectual property.

"This was a hot issue in the late '90s," Wagner said. "There are much more serious concerns (now): the instant messaging, the videoconferencing."

The Justice Department is nonetheless gearing up to defend the law at a trial set for October in U.S. District Court in Philadelphia.

The case spawned a high-profile debate recently when Google refused a government subpoena for documents the government sought as it develops its strategy.

Justice lawyers subpoenaed several leading search engines for information, apparently to study what information people seek — and find — online. They asked Google for 1 million sample queries and 1 million Web addresses in Google's database, according to court documents.

Google is fighting the subpoena, although primarily citing trade secrets, not privacy issues. Yahoo! and others are cooperating, saying the information they provided does not identify individual users.

"I think it's natural for people to think this is creepy, even though it's unlikely ... the Department of Justice would ever link that up with who I am," said John Palfrey Jr., executive director of the Berkman Center for Internet & Society at Harvard Law School.

The U.S. Supreme Court has twice granted preliminary injunctions that prevent the government from enforcing the Child Online Protection Act, known as COPA, until the case is tried.

Supreme Court Justice Anthony Kennedy wrote the 5-4 decision that upheld the latest injunction in June 2004 on grounds the plaintiffs were likely to prevail. He, too, questioned whether evolving technology had not substantially changed the issue since 1998.

He also noted that filters can block dubious Web material posted offshore, which the U.S. law could not target.

"Promoting the use of filters does not condemn as criminal any category of speech, and so the potential chilling effect is eliminated, or at least much diminished," Kennedy wrote.

The law, signed by then-President Clinton, requires adults to use some sort of access code, or perhaps a credit-card number, to view questionable material.

At least one earlier attempt by Congress to fashion a blanket online child-protection law, and about a dozen state laws, have previously been thrown out on First Amendment grounds, according to attorney Chris Hansen of the American Civil Liberties Union, which represents the plaintiffs. The Supreme Court has approved more limited measures.