"A Little Night Music" opens at the Trinity Theater, 36th and O streets NW, tomorrow night at 8 p.m. Other performances are Feb. 14, 20, 21, 27 and 28 at 8 p.m. and Feb. 22 at 2 p.m.

Weeknights at 7 o'clock, Joseph diGenova, newly appointed chief counsel and staff director of the Senate Rules Committee, locks his office door on the problems of federal election laws and campaign financing, and drives across town to grapple with another set of problems -- a virgin wife, an old mistress and a pistol packing rival.

A Senate lawyer by day, diGenova by night -- six nights a week for the past 2 1/2 months -- is a lead actor-singer in the Trinity Players' production of Stephen Sondheim's "A Little Night Music," which opens tomorrow night.

"If you work at it, if you get to the point where you're pretty good at it, theater is a good way to relax, to forget all the nonsense that happens during the day," says diGenova, ducking into a delicatessen for a sandwich to eat at the theater. "I couldn't do without theater. It's my golf game."

Inside the gilt-and-white 600-seat theater, the rest of the cast and crew is dining on Doritos, cookies and grapefruit juice and submitting bills for costumes and props to producer Paul Preston. Some, like diGenova, are what Preston calls "professional schizoids" -- lobbyists, electrical engineers and office managers by day and show people by night. But Christopher (Chip) King, who plays diGenova's pistol-packing rival, is a full-time opera singer, winner of last year's Metropolitan Opera regional auditions. And Jane Squier, who plays Desiree Armfelt, divides her time between paid dinner-theater engagements and unpaid community-theater stints.

"I've been begging the groups I belong to to do this show ever since it opened on Broadway," says Squier. The musical is based on a 1958 Ingmar Bergman film, "Smiles of a Summer Night." The musical version also was made into a movie, starring Elizabeth Taylor as Disiree Taylor, now a Georgetown resident and the wife of Sen. John Warner (R-Va.), is donating the hat and gown she wore in the movie to the Trinity production.

"Some of you may not have all of your costume until the preview Thursday night," says director Beau Stark ("As in raving mad"), pounding on the stage with a detached table leg as he tells the cast the schedule for what he calls "hell week" -- the last few days before opening night.

As the cast waltzes on stage, changing partners in a foreshadowing of plot developments, Stark explains that "A Little Night Music" is so difficult to do that the Trinity production is believed to be the first community theater staging of the work in the United States.

"Sondheim's music is extremely difficult. It's hard to get the voices you need," he says, stopping to direct the light crew's attention to a shadow across the face of Bari Biern, who plays the long-suffering wife of Count Carl-Magnus Malcolm, Desiree's jealous lover.

"Besides that, the show is a techincal nightmare. In the Broadway production they had the technology to make the sets fly on and off the stage. We don't, so our set designer, Jane Hayden, built this thrust so we can leave certain sets on all the time. Even so, in the second act we have to move a house front, a gazebo, 14 chairs and a topiary, all during the time it takes to sing one song," adds Stark, who by day grapples with the problems of adjustable rate mortgages for the Federal Home Mortgage Corp.

"The harmonic language is more advanced than in musical comedy -- it's almost an opera," explains musical director Marc Tardue, who is taking over at the piano from the rehearsal pianist in time for the first act finale -- an operatic sextet in which the singers perdict what will happen during their upcoming weekend in the country. "We had 115 people audition. This is a singer's musical. But we still couldn't cast Carl-Magnus, so I asked Chip King to do it, really as a favor."

When Act I ends, Tardue, Stark and choregrapher Rita Criggar have notes or pointers for everyone, and many of the actors have requests for the director of the prop crew. DiGenova wants something solid for himself and Glenda Lassiter, who plays his young wife, to lean on in the bed in which they sing a duet.

"We've been married 11 months, but I'm still a virgin -- defintely not typecasting," says Lassiter, who works as a marketing services manager and has no desire for a full-time theatrical career. "I don't function well without theater, but to me it's just an outlet. It doesn't offer what I get intellectually from my job."

DiGenova, who now has about nine hours before he has to be back at his desk in the Senate Office Building, once wanted a theatrical career but gave up the idea and went to law school.

"Politics is theater, too," he says. "And politics and theater are a delightful life blend. The character I play in the show, Fredrik Egerman, is a lawyer, too. I love that line of the grandmother's about lawyers. 'In my day we consulted lawyers in their offices. We didn't consort with their families.' That ought to bring down the house. This is Washington, so half the audience will be lawyers."