Congressional Republicans opened a new front in their war on Attorney General Janet Reno yesterday, blasting the Justice Department for a plea bargain offering a 12 1/2- to 15-year sentence to a notorious Boston mobster who has confessed to committing 20 murders.

House Majority Leader Richard K. Armey (R-Tex.) called on Reno to review the generous deal offered to hit man John Martorano, 58, who pleaded guilty yesterday to racketeering charges in federal court in Boston. U.S. Attorney Donald K. Stern gave Martorano the deal in exchange for his testimony against other New England gangsters, in a celebrated case that has deteriorated into a huge fiasco for the department.

"What signal does this administration send when it allows dangerous predators like Mr. Martorano to receive a token slap on the wrist after a lifetime of crime?" Armey wrote in a letter to Reno. "I hope you will share my dismay and take the necessary steps to prevent further unnecessary tragedies like this from being repeated."

Hill Republicans have tangled with Reno often in the past, over issues from gun-crime prosecutions to her refusal to seek an independent counsel to investigate campaign finance abuses. The relationship has reached a new low in recent weeks, with Republicans attacking Reno--but not FBI Director Louis J. Freeh--over revelations that FBI agents used potentially incendiary devices during the standoff with Branch Davidians near Waco, Tex.

The Boston case could be a gold mine for Reno's critics; by any reckoning, it has been a major law enforcement embarrassment, and it has already prompted the Justice Department to revamp its guidelines for the handling of informants. During pretrial hearings, it became clear that for years, FBI agents gave Martorano's co-defendants, Winter Hill Gang leader James "Whitey" Bulger and sidekick Stephen "The Rifleman" Flemmi, a virtual free pass to commit crimes in exchange for information on other organized crime leaders in New England.

Now the tables have turned, with Martorano getting leniency in exchange for dirt on Bulger and Flemmi, including their alleged involvement in murders in Massachusetts, Florida and Oklahoma. Bulger remains on the lam; some believe FBI agents tipped him off before his 1995 indictment. Flemmi has argued that he should be free as well because the FBI promised him immunity for his crimes; Judge Mark Wolf has essentially put the FBI on trial to investigate those claims.

In an interview, Stern defended his controversial deal with Martorano, arguing that Martorano's cooperation will serve the best interests of justice in the long run.

"This is distasteful, but it would have been more distasteful for us not to enter into this plea," Stern said. "We had a very stark choice: Leave these murders forever unsolved, or negotiate an agreement that will send Martorano to prison and may lead to the prosecution of others."

Yesterday, Wolf expressed concerns about the deal and reserved the right to scuttle it later. Even though Martorano pleaded guilty to 12 killings and has confessed to eight others, the plea could set him free as early as 2007 because he has already served five years in prison.

"We expect prosecutors to prosecute," Armey said in an interview. "I don't know what's going on over there."

A Reno spokeswoman said the attorney general has full confidence in Stern's decision, which was made in consultation with the department's criminal division and three state prosecutors. She called Stern "one of our finest U.S. attorneys."

The Martorano case has become a racial issue in Boston, and Armey told Reno yesterday that he was "alarmed to hear that Mr. Martorano may have sought out in particular black citizens." He was referring to a 1998 column by former Boston Globe columnist Mike Barnicle, who quoted a retired police officer saying Martorano "practically used black people for target practice." But of Martorano's 20 admitted victims, only four were black.

"To date, we haven't found a shred of evidence to support the charge that he used blacks as target practice," Stern said.