The Bush administration has brought 21 indictments in two years alleging that Web site operators and others crossed the line from acceptable smut to illegal obscenity, Olson told the court.

"With such a vast array of sites, there are so few prosecutions," O'Connor said. "It's just amazing."

COPA would make it a crime for commercial Web site operators to knowingly place material that is "harmful to children" within their unrestricted online reach. Violators can face six months in jail and civil and criminal penalties of $50,000.

The law is meant to go after the really bad guys, Olson argued. He suggested that the American Civil Liberties Union and other opponents of the law are crying wolf.

It's the government that is being unrealistic, ACLU lawyer Ann Beeson countered.

The law "criminalizes a depiction or description of nudity, or even a description of the female breast," Beeson told the justices.

The ACLU challenged the law on behalf of online bookstores, artists and others, including operators of Web sites that offer explicit how-to sex advice or health information. Among them is Mitch Tepper, whose Web site dispenses very specific instructions to help the disabled enjoy sex. One article he has posted online is titled "Handsfree Whoopie."

Tepper risks jail time if some prosecutor somewhere finds his material "harmful to minors," the ACLU argued. COPA gives no absolute definition of what is "harmful to minors," leaving that in part to "the average person, applying contemporary community standards."

The ACLU maintains that the community standards test is meaningless when applied to the far-flung Internet, but the Supreme Court ruled two years ago that that claim is not enough, on its own, to make the law unconstitutional.

The high court is expected to issue a more definitive ruling by summer.

The case is Ashcroft v. ACLU, 03-218.