To borrow a line from one of his Pulitzer Prize-winning productions, "A Chorus Line," Joseph Papp says. "I can do that." Now people tend to believe him.

The man who founded the New York Shakespeare Festival, and the man who is responsible for numerous award-winning on and off-Broadway hits proved that he can once again Tuesday night with a doggedly successful debut in his most personal production to date - as a cabaret singer.

What started out as a lark singing for a political benefit snowballed into a four-night engagement when the manager of The Ballroom, a tiny nightclub in Manhattan's Soho district, "I've always sung," said the 57-year-old closet cantor, "on the high holy days as a boy and now in the shower r the steam room, and this is a side of me I think people should see."

The house (75 capacity, maximum) was packed almost exclusively with friends and theater associates - those that have been, currently are, or would like to be employed by "the most vibrant force in American theater in the past 25 years." Elizabeth Swados, author of his current big moneymaker, "Runaways," sent ginseng and honey to his dressing room.

He took to the stage looking like an anxious maitre d' with tousled hair and sweaty palms which he nervously wiped on the front flaps of the Theoni V. Aldredge-designed tuxedo before crooning his way through a musical compendium of his formative years: ages 5 through 15.

Opening with Rodgers and Hart's "What Do You Want With Money" Papp went through the collage of tunes "my mother, my father, my sisters and brother and the radio taught me." The generous mix included excerpts from the Rosh Hashanah service, and "Nagasaki" ("where the girls go whicky wacky woo"). The depression-ear economic policies of Harry Warren were campily sold with "I Found a Million-Dollar Baby"; Irish politics were stoically served with the ballad "Kevin Barry."

Accompanied by a five-piece orchestra in black tie and tails, he tried his hand at various styles of singing. While Ted Lewis, Al Jolson and Bing Crosby all managed to wend their way through the performance, it was apparent that he was favoring his Frank Sinatra side as he hit a casual high on the reprise rendition of "Hallelujah I'm a Bum Again."

How would the producer rate the singer's performance? "It's like the character in Dostoevski's novel who is asked how he would rate his life. I have nothing to compare it to."

While Clive Barnes, reviewing for The Daily Metro, called it a qualified "triumph" and labeled him a "Not-Yet-Ready-for-Prime-Time Singer," he was as generous with his applause as the rest of the crowd. The decided hit of the evening was Papp's tuxedo flaps-up, coming-in-for-a-landing rendition of "Brother Can You Spare a Dime."

That was a particularly conscious effort on the part of the performer. "I had to make it up to them on that one," he said backstage among the congratulatory flowers in his dressing room. "Since I forgot the words to the song just before. 'Nagasaki.' There isn't any scat singing in the original version."

The intrepid entertainer wiped the rivulets of sweat from his brow and assessed the noble experiment. "Someone asked me if this is the most horrible moment of my career. I don't think so, it's just that the best part of the whole thing has been working up to this point. The rehearsals are fantastic - singing four hours a day for the past two weeks has been terrific and I've been totally possessed by these songs. The performances are my exorcism of sorts."

"It's the old Everest theory, I suspect," said Barnes. "I imagine he did it because it was there. And of course, mind you, Joe Papp has a tremendous ego, but he's always one for extending himself into new areas."

Though Papp has managed to get his act together, he won't be taking it on the road. "I don't plan to do this again. I've got another career to keep me busy," he added, hedging the subject, "though if someone asks me to sing at a wedding or for a good cause I just might accept."