The District of Columbia will be the first jurisdiction in the country to test a new federal law imposing mandatory minimum 15-year sentences on certain repeat criminals, U.S. Attorney Joseph E. diGenova said yesterday.

"We're going to be the pilot office for the United States," said diGenova, who prosecutes all serious local and federal crimes in the city under the District's unique criminal justice system.

Sen. Arlen Specter (R-Pa.), chairman of the Senate Appropriations subcommittee on the District, predicted that use of the new authority will be a strong deterrent to crime in the city, will result in more guilty pleas by repeat offenders in D.C. Superior Court, and will help clear out a backlog of cases in that court.

Under the crime bill passed at the end of the current Congress, federal prosecutors can take over from the local prosecutor any case involving use of a gun if the defendent has been convicted three times previously of robbery or burglary.

If the defendent is convicted of that fourth crime in U.S. District Court here, the 15-year mandatory sentence will be imposed with no possibility of probation or parole. If the same person was convicted of the same crime in D.C. Superior Court, a significantly lesser sentence with the potential for parole would be likely.

The U.S. attorney's office in recent years has shifted almost all of its criminal prosecutions to D.C. Superior Court and has tried only the most serious drug distribution and fraud offenses in U.S. District Court.

DiGenova said D.C. was chosen to test the program because he can put it into effect faster than can other jurisdictions since he is both local and federal prosecutor and there would be no need to coordinate two offices.

The U.S. attorney said his office now is evaluating cases that have come into his office since passage of the law to see how many fit the criteria, and will start sending some to U.S. District Court rather than Superior Court when they are found.

"I'm anxious to see Washington as the model city on the new career criminal bill," said Specter, who drafted the provisions.

Specter said that as district attorney in Philadelphia he had about 500 of these types of career criminals on his docket at any one time. If he could have sent five of these over to the U.S. attorney for prosecution under the 15-year mandatory sentencing law, Specter said he could have gotten guilty pleas in the local court for perhaps 400 of the others who would not risk being convicted in federal court.

"There is a great potential for clearing the dockets, and it gets these guys off the street," said Specter, who held hearings yesterday on career criminals. It also could mean longer sentences from Superior Court judges for repeat crimes, he said.

"It the 15-year sentences certainly can be a deterrent," said diGenova. But he said he could not tell yet if the new authority would clear up the Superior Court backlog because he does not know how many cases qualify for the mandatory sentencing treatment.

Specter also said he was displeased at how slow the city has been to put tough new probation rules into effect. The senator has pushed for new parole and probation rules that would require revocation of probation or parole for persons found with drugs in their system.

The city has proposed new rules but they have not gone into effect. "They haven't gotten it done," said Specter.