It's been a quarter century since the US Supreme Court declared an entire category of speech unworthy of First Amendment protection.

On Tuesday morning, the Obama administration will ask the high court to carve out a new exception to the Constitution's free-speech mandate. Government lawyers say the action is justified in the name of a noble cause – preventing cruelty to animals.

At issue in US v. Stevens is whether Congress overstepped its authority when it passed a 1999 law barring the creation, sale, or possession of any depiction of animal cruelty with the intent to distribute and sell it.

The law was aimed at blocking a small but growing market in underground sexual-fetish videos that involve dominatrix women who step on and kill small animals. By one estimate in 1999, these so-called "crush videos" represented a million-dollar market.

Rather than focusing exclusively on crush videos, Congress wrote a statute criminalizing a wider swath of conduct involving "animal cruelty."

Free-speech advocates warn of the dangers of roping off an entire section of images and ideas from public discourse. There is a value to protecting unpopular, dangerous, even repulsive speech in the marketplace of ideas, they argue. Would someone seeking to sell a Spanish bullfight video in the United States be subject to a fine and prison term?, they ask.

Animal rights activists counter that the government has a compelling interest in preventing the inhumane treatment of animals. They stress that videos depicting such mistreatment are obscene and have no social value.

The Supreme Court last declared a category of speech to be unprotected by the First Amendment in 1982: Child pornography did not deserve constitutional protection, it said.

Other forms of speech declared unprotected include: fighting words in 1942, threats in 1969, speech inciting illegal activity in 1969, and obscenity in 1973. In addition, there is no right to make libelous statements.

A key question in the Stevens case is whether animal cruelty rises to the same level of government concern underlying these prior rulings. All 50 states have enacted laws against acts of animal cruelty and obscenity.

The case before the high court involves an author and documentary producer named Robert Stevens who was convicted of violating the law by selling three videotapes about pit bulls to undercover agents. He was sentenced to three years in prison. A federal appeals court threw the conviction out after declaring the law violated the First Amendment.

One film showed pit bulls catching wild boars during hunting trips. Another showed pit bulls engaged in a dogfight in Japan, where such fighting is legal. A third film included dogfight footage shot in the US in the 1960s and '70s.

Mr. Stevens has said he included the dogfight and hunting depictions not to promote dogfighting, but to demonstrate aggressive characteristics that distinguish the pit bull breed from other dogs.

The statute outlaws only a narrow category of speech – "bloody spectacles of vicious animals forced to fight to the point of exhaustion or death," Solicitor General Elena Kagan writes in her brief. In addition to crush videos, the law bars depictions of dogfights, hog-dog fights, and cockfights. "Such images are far removed from the free trade in ideas that the First Amendment was designed to protect," Ms. Kagan says.

Stevens's lawyers disagree. "The notion that Congress can suddenly strip a broad swath of never-before-regulated speech of First Amendment protection and send its creators to federal prison, based on nothing more than an ad hoc balancing of the 'expressive value' of the speech against its 'societal costs' is entirely alien to constitutional jurisprudence and a dangerous threat to liberty," writes Washington lawyer Patricia Millett in her brief to the court.

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