The witness backed into his reply, opening with a few statistics, adding a few asides, and then in a comment that sounded almost parenthetical, reversing a long-held position.

District Mayor Marion Barry was telling Sen. Arlen Specter (R-Pa.) what he had wanted to hear for some time: Maybe it wouldn't be such a bad idea to build more detention facilities to house an expected increase in the District's prison population.

Specter, known for a relentless approach to his Senate goals and squash game alike, this year started his second two-year term as chairman of the Senate Appropriations subcommittee on the District. A new prison for the city was at the top of his agenda, and if one is finally built, it will be largely because this freshman senator from Pennsylvania, a former Philadelphia district attorney, wants one.

The District is the only city whose spending is under the control of Congress, and hearings on next year's budget began this month. Since the city's affairs rank near the bottom of concerns for most senators, the subcommittee chairman generally dominates how D.C fares in the Senate.

While many chairmen have seen the D.C. committee as little more than freshman hazing, Specter has thrown himself into the job with gusto. He has seized the assignment as an opportunity to try out his ideas for revamping the criminal justice system, using the District as his laboratory.

The job also has given Specter, who conducts hearings with the cool precision of a former prosecutor, a forum for exploring a wide range of urban issues, including drug abuse, truancy and homelessness.

"He has spent a great deal of time on us," said D.C. Budget Director Betsy Reveal. "He is the first appropriations chairman that has been willing to put in federal funds for his special interests. That is quite a significant change."

When the District's appropriations bill came up before the full Senate Appropriations Committee last year, Chairman Mark O. Hatfield (R-Ore.) noted that the D.C. subcommittee chairmanship was traditionally seen as "the bottom rung of the totem pole assignment." But Specter, he said, had not viewed it that way, taking it seriously and "equal to any other."

In his first two years as chairman, Specter pushed the city into adopting tougher parole and probation guidelines, got seven new judgeships on the D.C. Superior Court, initiated a school truancy program and started a pilot educational program at the city's Lorton Reformatory that he wants to make a "model for the nation."

"I consider it a very interesting assignment, to play out some of my ideas in the criminal justice system," Specter said. "When I was able to add $30 million for the Lorton program, Office of Management and Budget Director David Stockman hated it, but he couldn't get the president to veto an appropriations bill over $30 million."

In the final days of the last Congress, he took it upon himself to push the White House toward a successful resolution of serious problems with the District's home rule charter, a complex issue that was not a financial matter and originally was before another committee.

To his admirers, Specter, 55, is brilliant, hard-working, disciplined, determined and ambitious. To his critics, he is bright but compulsive, uptight, abrasive and opportunistic. He is known on Capitol Hill as a demanding boss. Those who appreciate his sense of humor say it is subtle and dry and easily misunderstood.

His leisure activities tend to meld with his professional life. Asked what he does in his spare time, Specter said he likes to read biographies of some of his Senate colleagues. Among his closest friends, he says, are senators he plays squash with or travels with on business.

"You get the impression that he's working all the time, or thinking about it," said Sen. John W. Warner (R-Va.), one of Specter's frequent squash partners. "He is a tirelessly hard worker."

Asked if he is a workaholic, Specter gives a typically wry answer. "I'm a squashaholic," he said, alluding to his regular 7 a.m. game. "I do a lot of things with intensity." As a child, Specter said, he had a mania for baseball statistics and could recite all of Ty Cobb's records.

A trim man with curly dark hair and basset hound eyes, Specter smokes an occasional cigarette but apparently has the self-discipline not to get addicted. "Sometimes he smokes a cigarette just to slow down," said one top Specter aide. "It's the only time he ever sits still."

The senator's wife, Joan Specter, is a member of the Philadelphia City Council. She made a name for herself years ago with a well-known Philadelphia pie company that she started and still runs.

Juggling two demanding careers is not easy for the Specters. He goes home to Philadelphia on weekends, and she joins him at their Georgetown condominium one night a week, taking the 4 p.m. train down and the 7 a.m. train back.

Specter, who is in the fifth year of his first term, is up for reelection in 1986. Pennsylvania politicians in both camps say he's unlikely to have close competition. Major pluses, they say: Specter's drive and regular contact with his constituents. Major negative: an aloof image.

"He doesn't have what you would call a common touch, although he likes to think so," said one Republican political analyst familiar with Specter's 1980 campaign. "He comes across as a Philadelphia lawyer stuffed shirt.

"In private, he's like he is in public," the analyst added.

As a member of the Senate Judiciary Committee and chairman of its juvenile justice subcommittee, Specter has held hearings on Bernhard H. Goetz, the purported New York subway "vigilante"; on Cathy Crowell Webb, who recanted her rape story that convicted a man, and on fugitive Nazi war criminal Dr. Josef Mengele. The senator also has been in the spotlight on school prayer (he was against the proposed constitutional amendment) and the MX missile debate (he cast a key vote for the system).

"Every day it's another issue," said Rep. Bob Edgar (D-Pa.), who plans to challenge Specter next year. "He's spread thin."

Specter gets favorable reviews from a perhaps surprising source: Mitch Snyder, radical advocate for the homeless and frequent bane of officialdom.

"I like him," Snyder said of Specter. Snyder got into a testy exchange with Specter at a congressional hearing when he called a Philadelphia shelter a "rat trap," a characterization Specter challenged. But Snyder called his own shelter at Second and D streets NW a rat trap, and later that day he learned that Specter had dropped by there for an unannounced inspection.

"I think he was personally appalled and concerned," Snyder said. "I don't think he has any clear sense of what to do about it, but he is groping with it."

Specter, whose father was a Russian-born Jewish immigrant, grew up in Russell, Kan., the same home town as Senate Majority Leader Robert J. Dole (R-Kan.). He says his interest in Washington started early, probably with his father's fury over World War I veterans not getting bonuses.

A graduate of the University of Pennsylvania and Yale University Law School, Specter went into private practice in Philadephia before becoming an assistant district attorney there in the early 1960s. Specter developed a reputation as a tough prosecutor after getting corrupt Teamsters Union officials sent to jail.

He received national recognition in 1964 as staff attorney on the Warren Commission, where he was considered principal author of the commission's much-disputed "single-bullet" theory on the Kennedy assassination. The theory holds that only one bullet hit President Kennedy and Texas Gov. John Connally -- and that just one gunman was involved.

Once a liberal Democrat, Specter switched parties in 1965 to run against his former boss for district attorney. He won, and he soon acquired a reputation as a publicity-hungry official who launched numerous high-profile investigations and held frequent news conferences.

But at least one reporter on the scene then says Specter got a bad rap as a publicity hound.

"We pressured him to do it," said Dave Racher, who has covered Philadelphia city hall for the Philadelphia Daily News since the early 1960s. "I thought he was one of the best DAs we've ever had in this town . . . . He wasn't afraid to take on the heavyweights."

Running as a Republican in a heavily Democratic city, Specter narrowly lost a bid for mayor of Philadelphia in 1967. But after winning a second term as district attorney in 1969, the one-time rising star of Philadelphia politics went on a losing streak.

He failed to win a third term as district attorney in 1973, lost a Senate primary race to John Heinz in 1976 and unsuccessfully sought the gubernatorial nomination in 1978.

After returning to private practice, he ran again for Senate in 1980. This time he won. Today, Specter says he enjoys the Senate and wants to stay there.

"I found a lot more frustrations staying awake in the afternoons practicing law and making a lot of money," said Specter, whose Senate salary of $75,100 is dwarfed by the $200,000 to $250,000 that a spokesman said Specter made in private practice. "You can do a lot more in the United States Senate than you can in a Philadelphia skyscraper charging $300 an hour."

Not a compromiser by nature or training, Specter acknowledges he has had to change his style since leaving the clash of the courtroom for the civility of the Senate.

"It's a very different role and it takes getting used to," he said recently. "I think occasionally I'm too confrontational, but I'm trying other things."

He has not had to worry about maintaining order in the District subcommittee, because most of the time he has been alone on the dais. In the past, his proposed budget has been circulated to other members and approved with only minor additions.

Specter's D.C. budget hearings focus on areas of particular interest to him, with a wrap-up overview from the mayor.

The House subcommittee takes a different approach to the budget, holding hearings on each agency, from the Office of the Mayor to the Washington Aqueduct. The chairman, Rep. Julian C. Dixon (D-Calif.), and other panel members ask detailed questions about the inner workings of each agency but generally steer away from pushing for policy changes.

So far Specter has had no problem getting House support for his pet projects.

Specter does not limit his questioning of District officials to budget season; he frequently calls them to the Hill to discuss a variety of issues. Last year, he publicly questioned Barry three times in as many months on why the educational program at Lorton was not up to speed.

While Specter and Barry say they have a good working relationship, there is an inherent tension between the city's goal of complete autonomy and its desire for the federal largesse Specter has been willing to provide.

"I don't like it . . . but if I have to do it, I have to do it well," said Barry of his trips to the Hill to justify his requests. "I don't resent it; why should I?" he asked, and he referred to Specter's ascension to the chairmanship as the time Specter "became my chair."

Barry probably would have come around eventually to the need for a prison because of crowding and political pressure, city budget director Reveal said, but "it would have taken longer absent the kind of leadership Senator Specter has shown on the issue."

Another knowledgeable city official points to the inherent power of the chairman's position and puts the rationale behind the mayor's change of heart more bluntly: "You can't slap him [Specter] in the face . . . when you've got your arm twisted behind your back.