The D.C. Department of Corrections opened a new permanent dormitory unit at Lorton Reformatory yesterday to help ease crowding at the D.C. Jail, where the return of 55 inmates from Pennsylvania swelled the population close to the court-ordered limit, corrections department sources said.

Two buses carrying the inmates from the 268 Center in Cowansville, Pa., where they were sent Friday night in an attempt to find alternative housing for city prisoners, arrived at the jail about 1:45 p.m. after an eight-hour ride from the private prison, about 50 miles north of Pittsburgh.

The prisoners had been ordered to leave the center by a Pennsylvania state judge, and shortly after their return Corrections Director James F. Palmer released a statement blaming the debacle on "political cross fire" between Pennsylvania's Sen. Arlen Specter and Gov. Richard L. Thornburgh, both Republicans.

Spokesmen for Specter and Thornburgh said last night that both were opposed to District inmates being housed at the 268 Center and denied that there was any "cross fire" between them.

D.C. Mayor Marion Barry, in a brief interview, said that "Mr. Palmer overstated the situation. As far as I am concerned, there is no conflict between the governor and the senator on this."

Thornburgh, who is in the final year of his second term and cannot succeed himself, considered challenging Specter, who is up for reelection this year, but decided against it because of fears a divisive primary would cause the GOP to lose the seat. The governor blasted the District last week for sending the inmates to Pennsylvania without notifying state officials of the transfer.

According to Palmer's statement, "In correspondence from Specter's office, we were encouraged to look into the 268 Center as a possibility for temporary relief at our correctional facilities."

Dan McKenna, Specter's press secretary, said last night that "The senator doesn't know what Palmer is talking about . . . . He knows nothing about any correspondence."

"I do recall getting a letter, but the exact contents I don't recall," Barry said, adding that Specter has been a "friend of the District," especially with regard to prison issues.

Specter, chairman of the Senate Appropriations Committee's subcommittee on the District, was influential in getting congressional approval of a $30-million appropriation for a new prison in the city.

In an interview earlier yesterday, Specter said he will hold hearings, possibly as early as next week, to search for solutions to the city's prison crisis.

"I respect home rule very much, but I think it's a fair area of inquiry at this juncture," he said. "I think the District subcommittee has an interest, and certainly a Pennsylvania senator does, too, given the recent efforts to send D.C. inmates to Armstrong County in Pennsylvania."

He said he was concerned about reports of prisoners sitting in buses for long periods because of no room at the jail, the slow pace of progress in the construction of a new prison in the city and the effect jail overcrowding may be having on judges' sentencing policies.

Yesterday's return of the 268 Center inmates to the D.C. Jail apparently pushed the prisoner population, which was 1,662 at 4 a.m. yesterday, close to the 1,694-inmate limit. About 4 p.m. inmates were being held on buses outside the jail because their presence inside would have pushed the population over the limit.

Sources said the backup occurred because the Corrections Department was slow in transferring 50 to 60 inmates to a new full-time dormitory at Lorton's Occoquan I facility in southern Fairfax County.

The dormitory, which previously was reserved for weekends-only inmates and was used only sparingly during the week, was reclassified yesterday as a full-time housing unit for 75 inmates, raising the inmate capacity of Occoquan I from 436 to 511, the sources said.

On the last two Fridays, the Corrections Department has had to cope with record numbers of people sentenced by judges to spend the weekend in prison. On March 7, 42 weekenders whose admittance into the jail would have forced the population over the limit were held on buses outside the facility for up to 10 hours before being sent to the weekender dormitory at Occoquan.

Sources said city officials have asked for permission to renovate the basement cell block in the old D.C. Superior Court Building B, 409 E St. NW, so that it can hold the weekenders.

That cell block, which has no showers, sinks or beds, is used only occasionally by D.C. police as a holding facility when large numbers of people are arrested, such as at a demonstration.

The building is controlled by D.C. Superior Court, and the decision on whether to allow it to be made into a housing unit for weekenders will be made by the Joint Committee on Judicial Administration, a group of five judges who have yet to rule on the request, sources said.

Meanwhile, lawyers with the American Civil Liberties Union, which represents inmates in the 15-year-old D.C. Jail lawsuit that resulted in the population ceiling, filed papers yesterday opposing a motion by residents of a Capitol Hill neighborhood who want to enter the lawsuit in an attempt to have the population ceiling relaxed.

Attorneys for the neighbors who are opposed to the opening of an emergency jail facility at 525 Ninth St. NE have asked U.S. District Judge William B. Bryant to stay his order setting the ceiling to provide a "cooling-off period."

The period, the group argues, will allow D.C. officials time to find short-term solutions to the crowding crisis that would not require opening of emergency jails in residential areas.