WASHINGTON — A two-decade-old legal fight over protests outside abortion clinics returned yesterday to the Supreme Court for the third time, with anti-abortion groups arguing they should be allowed to demonstrate without fear of lawsuits and large penalty judgments.

Two years ago, the high court said there was no basis for using federal extortion and racketeering laws to ban demonstrations by anti-abortion leader Joseph Scheidler and others. An appeals court subsequently ruled that a nationwide injunction may be supported on other legal grounds, and the protesters brought the case back to the Supreme Court.

In lively questioning, most of the justices, including new Chief Justice John Roberts, seemed intent on finding a way to issue a narrow ruling that would bring the case to a close.

Justice John Paul Stevens appeared willing to concede the possibility that the Court in 2003 had "overlooked" four jury findings involving threats of violence by the protesters and their possible impact on the injunction.

But Justices Antonin Scalia and David Souter questioned why the 7th U.S. Circuit Court of Appeals had not ended the case with the high court's reversal and lifting of the injunction in 2003.

Instead, the appeals court asked a trial judge to determine whether the injunction could be supported by charges that protesters made threats of violence absent a connection with robbery or extortion.

Justice Stephen Breyer said such an approach would create a massive change in federal law covering crime and labor activity.

Breyer's questions highlighted concerns of social activists and the AFL-CIO who worry their efforts to change public policy or agitate for better wages could be targeted by similar suits and injunctions.

Such a change would "transform virtually every threat of violence anywhere in the United States into a serious federal crime," Breyer said, "and at least would make a major change in threats of violence on the picket line."

The legal battle began in 1986, when the National Organization for Women filed a class-action suit challenging tactics used by the Pro-Life Action Network to block women from entering abortion clinics.

NOW's legal strategy was novel at the time, relying on civil provisions of the 1970 Racketeer Influenced and Corrupt Organizations Act, which was used predominantly in criminal cases against organized crime. The lawsuit also relied on the Hobbs Act, a 55-year-old law banning extortion.

A federal judge issued a nationwide injunction against the anti-abortion protesters after a Chicago jury found in 1998 that demonstrators had engaged in a pattern of racketeering by interfering with clinic operations, menacing doctors, assaulting patients and damaging clinic property.

The Supreme Court ruled that because the protesters had not extorted money or valuables from the clinics, there was no basis for a racketeering violation or the injunction. But the appeals court found that the high court had not considered fully four counts of making a threat of violence that might be enough to support the ban.

Lisa Blatt, an assistant solicitor general, argued that no defendant had ever been convicted of an act of violence under the Hobbs Act that didn't relate to a robbery or extortion.

The cases argued before the Court yesterday were Scheidler v. NOW, 04-1244, and Operation Rescue v. NOW, 04-1352.