D.C. Mayor Marion Barry will have to build more jail facilities, an idea he has strongly opposed, said Sen. Arlen Specter (R-Pa.), chairman of the Senate subcommittee that oversees District appropriations.

"I'm not going to press him on it a little, I'm going to press him on it a lot," Specter said yesterday at a luncheon with Washington Post reporters and editors.

"We have to have expansion and new facilities," he said. "I do not believe the mayor will be adamant against it. When faced with the evolving factual picture, Mayor Barry will agree we need additional jail facilities."

The city faces an extreme problem of jail overcrowding, which some predict will get worse with new mandatory sentencing requirements and proposed toughened rules on revoking probation and parole for rearrests or evidence of drug use.

The District's prison facilities at Lorton are designed to hold a maximum of 3,503 inmates, and the D.C. Jail has a capacity of 1,378. This month the combined capacity has been exceeded by more than 1,250.

The mayor and other city officials are under court order to relieve crowding, but so far the mayor has not been moved to try to build a new prison.

At a news conference last month, Barry said the city would manage its overcrowding problem but would not build any more prison facilities beyond a 400-bed addition at Lorton that opens next year because judges would just fill them up.

He suggested instead that one way to reduce the prison population would be to keep persons convicted of nonviolent crimes out of jail, sentencing them instead to make restitution to their victims or to perform community service.

D.C. City Administrator Thomas Downs, acting mayor while Barry is on a three-week visit to Africa, said yesterday that the city has increased its prison capacity by 33 percent since 1980, adding 1,887 cell spaces at a cost of $28.5 million.

"We have done more than any other jurisdiction in the United States to increase our prison capacity," Downs said. "We have a right to say 'Let's look at some alternatives.' "

Specter said he expects to see a change soon in the membership of the D.C. Parole Board, which he has criticized sharply for letting offenders out of jail too soon and for not revoking parole of those rearrested or found with drugs in their system.

He declined to name members he thinks the mayor will replace, but said he is "disturbed that it hasn't come about by now."

A recent analysis found that, of about 300 persons in the District arrested while on probation, parole or prerelease and not detained, 90 were arrested for a third time after their rerelease, Specter said.

The senator said that his agenda on District matters next year will include concentration on the school system, particularly truancy problems, and an effort to do more in the area of mental retardation and homelessness.

Congress still has oversight of the District government and approves its budget each year. In the past, Congress has concentrated much of its attention on public safety and criminal justice, and at times it has forced the city to take actions such as increasing its complement of police officers and firefighters.

Specter acknowledged the difficulty in finding a place for a new prison for D.C. offenders.

"We had in Pennsylvania authorization for a prison in 1965, and it has not yet been built. The only place where we could get agreement within the state was in New Jersey," he said.

Specter agreed with Barry that more nonviolent offenders should be kept out of jail.

It would be better to concentrate on murder cases and other serious crimes and divert persons accused of less serious crimes such as shoplifting or petty larceny, he said.