IT'S settling down and darkening," says J. Patrick Raftery. "I don't think it will start going up, and I certainly hope not."

He is talking about his most precious possession -- a baritone voice that has taken him, in a few years, from high school and community productions, of musical comedy into the operatic major leagues. At 23, Raftery has just finished an engagement with the Washington Opera, singing the title role in "The Barber of Seville," his first starring role with a major company. But he is on his way to sing Gounod in Chicago, Verdi in San Diego and Bizet in Paris. At an age when most operatic singers are still in school and wondering whether they should try to build a career, Raftery is already well established and completely confident of what he will be doing in the next decade or two.

A few times in a century, an opera singer manages to break through to stardom at around Raftery's age -- and when it happens, it makes operatic history. It usually happens to sopranos, the voices that mature youngest. The most memorable cases in the last generation were Roberta Peters, who made her debut at the Met (as a last minute subsitute) when she was 20, and Maria Callas, who actually turned down a Met contract, after a spectacular audition, when she was 22. Callas did not make her debut until two years later, and many stars take much longer. Beverly Sills, for example, spent years working her way up through the ranks before she began singing with the New York City Opera and did not appear at the Met until she was in her mid-40s. The singers most comparable to Raftery are probably Robert Merrill and Sherrill Milnes, who were considered very young when they began singing with major companies in their late 20s.

At 23, Raftery, is already a seasoned veteran in supporting roles and just beginning to take the spotlight. Last year, he sang Schaunard in "Boheme" with Pavarotti in San Diego. This year, he will have the role of Mercutio in the Chicago Lyric Opera's production of Gounod's "Romeo and Juliet." "And next summer in San Diego," he says with a chuckle, "I am going to be king for a day in Verdi's 'Un Giorno di Regno.'"

His is the kind of voice that matures latest and lasts longest. It is in excellent form now, and he uses it with an intelligence that promises phenomenal results when it begins to reach its full richness in seven or eight years. One veteran opera observer says, "He is almost certainly destined to be the most spectacular bass-baritone since Milnes and his career will undoubtedly take him to every major opera house in the world."

Raftery will modestly concede that talent had something to do with his accelerated career, but he says that "a lot of it was luck -- being in the right place at the right time and meeting the right people and being given good, healthy things to sing."

A native of Washington, he grew up in Bethesda -- where he still lives with his parents -- and graduated four years ago from St. John's Military Academy on Military Road. "I never did get a college degree," he reflects, "although I studied for three years at the Boston Conservatory and a year at Julliard.It was ridiculous. I was turning down jobs performing so I could go to school to learn how to perform."

He had learned in his teen's "I've been acting since I was 12," he says, "and singing since I was 15. I started to study singing because I was hurting myself, singing incorrectly in musicals." Before he began work on Verdi and Puccini, he had accumulated credits in such shows as "West Side Story," "Pajama Game," "Oliver," "Show Boat," "Brigadoon" and "The Music Man."

"I sang in about 20 school and community productions in three years, beginning when I was 15," he says. "I had mostly principal roles, but I also sang in the chorus. I was a tenor then and could hit a high A-flat, and sometimes I was the only tenor in the chorus because I was loud enough all by myself to balance the rest of the chorus. I guess the most classical role I sang during that period was Ralph Rackstraw in 'H.M.S. Pinafore.'

"Then I had to start studying to protect my voice. I wanted to take a course in musical theater. When I began studying with David Beckwitt of Montgomery College, I told him I absolutely did not want to sing opera. mI began studying in September. The next spring, they were doing 'Carmen' and needed an Escamillo. I auditioned for it at 17 and beat out a lot of college guys, and it's been downhill from there -- opera all the way."

Raftery's conversion to opera is still so recent that he can recall the first operatic record he bought (Leontyne Price in "Carmen") and the first opera production he saw: the Bolshoi's "War and Peace" at the Kennedy Center. "I bought the presidential box for that performance, because I wanted to do it right," he says. "Since then, I haven't seen all that many operas. I spend so much time in the theater that when I have a day off I want to do something else -- go to a movie or go out and have a few drinks with friends."

After his first operatic role, as the bullfighter in "Carmen," Raftery (who was then a high school senior) was taken by his teacher to audition for the Wolf Trap apprentice program. Frank Rizzo, dramaturg of the Washington Opera, recalls that audition: "Kay Shouse asked me if I would audition the 17-year-old son of a friend of hers, and I told her that I would, although an audition at that age wouldn't tell me much. Then in te audition, where he really opened up, my reaction was "My God, what have we here?' He was too young for the apprenticeship program, so we let him audit the course."

Besides Rizzo -- who "has been my confident and friend and advisor for five years" -- Raftery's stay at Wolf Trap introduced him to other key people who have guided his career: John Moriety, who was his voice teacher for three years at the Boston Conservatory; and stage director Lou Galterio, who directed the Washington Opera's current productions of "The Barber of Seville" and "Postcard From Morocco."

Between engagements elsewhere, Raftery is likely to be one of the mainstays of the Washington Opera in the years ahead. "We will certainly have him back in 1982 and possibly next season," says Rizzo. "We have taken options on all his available time that fits our schedule through 1984. It seemed pointless to go beyond that."

Raftery is building his career carefully, choosing roles that suit the current state of his voice ("very lyric, young repertoire," he calls it) and looking ahead to roles he hopes to tackle when the voice is ready. "I want to do Macbeth, and 'Bluebeard's Castle' and Wolfram in 'Tannhauser,'" he says, "But more than anything else I look forward to doing Nick Shadow in 'The Rake's Progress.' That's my favorite opera. But I'll have a long time to wait for most of this -- maybe 10 or 15 years. You don't sing Bluebeard until you are doing Scarpie, and I won't be ready for that for a while." Meanwhile, he is concentrating on earlier literature that is less taxing for a young voice. The next composer on who he will concentrate may be Mozart, "my favorite composer."

"I'm negotiating with someone right now about singing Leporello," he says. "I probably won't do Don Giovanni for a few more years because I have to come to terms with the character. But Massetto, Figaro and the Count are all possibilities, and I would kill for a chance to sing Papageno."

After he has sung all these roles, at the age of 35 or 40, Raftery looks forward to interrupting his career, going back to school, and perhaps teaching. "I would love to teach Romance languages -- Italian and French -- and English literature," he says. "I would love to teach Latin." fBut he probably will not want to leave the world of opera and he even dreams of opera yet unborn: "I think the opera I most want to see composed is "Hush, Hush, Sweet Charlotte,' with a coloratura in the Bette Davis role, a spinto soprano in the cousin's role that was played by Olivia De Havilland, the doctor sung by a baritone, and a tenor taking the part of the old lover who was murdered.

"It has everything -- even a peasant chorus and a character mezzo role in the maid who was played by Agnes Moorehead. The scene that Bette Davis does backing down the stairs could be one of the great mad scenes in opera."