Gershwin classic; Radio City's 'Porgy' is half Broadway, half opera

George Gershwin's ''Porgy and Bess,'' though conceived as an opera, has generally found a more comfortable home on Broadway. At its best it is a desperately moving work, and its latest incarnation falls neatly between opera and Broadway. To see it at Radio City Music Hall (through May 15) is a unique experience - that huge palace adds a grandeur to the proceedings that even a great opera house cannot quite capture.

In fact, this production is a variation of the version first seen at the Houston Grand Opera, then seen on Broadway and on a nationwide (and European) tour during the US Bicentennial. Now, though, it has a new choreographer and set designer.

Radio City is a huge space - 5,800 seats - with a huge stage. Thus the scale is opera, not Broadway, and that alone makes this ''Porgy and Bess'' something different. The performers deftly tread that line, too.

''Porgy'' is a uniquely American product from one of America's legendary songsmiths. It is a brilliant synthesis of much that is indigenous to the nation's musical voices - folk, jazz, musical comedy, ''serious'' music. The controversy over where ''Porgy'' belongs will rage long, but I unhesitatingly place it behind an opera-house proscenium, cast with rich operatic voices, and with a full 96-piece orchestra in the pit. That will come in a few seasons at the Metropolitan Opera.

Meanwhile, we can content ourselves with this version at Radio City. It restores anew one's admiration for the score - a rich panoply of fabulous tunes, superb arias, duets, ensembles, thrilling storm music, everything an opera must have. It is forcefully orchestrated throughout most of its 2-hour-45-minute duration. Occasionally the threads that seam set pieces together show more than they ought to, but that is part of the work's charm. The forward motion of the plot and the anticipation of the next scene, the next Gershwin melody, make the listener ignore small flaws of this kind, which are present in all operas that are not Mozart or late Verdi.

The production at Radio City moves smoothly under Jack O'Brien's direction, even if, on occasion, he opts for motion rather than pointed character revelation. The principal roles have as many as four alternates. In the two casts I saw, there were numerous standouts. Larry Marshall, one of the holdovers from the last go-around, continues to be a spontaneous, suave, insinuating Sportin' Life. Alexander Smalls sings Jake's music with firm voice and expert characterization. Shirley Baines is a riveting Serena vocally and histrionically. Gwendolyn Shepherd and Loretta Holkmann both make the small role of Maria stand out. Luvenia Garner's lovely voice suits ''Summertime'' to a tee, despite the languid tempo conductor John Miner set for her.

Bess needs a performer of remarkable presence, a superior voice, and a curious sort of vulerability. Daisy Newman offers the sumptuous voice, some of the presence, while Henrietta Elizabeth Davis offers the presence and some of the vulnerability. As the evil influence in her life, it would be hard to imagine a more terrifying Crown than Gregg Baker - a blazing, fierce presence, an exceptional actor and a fine singer.

As the hope in Bess's life, the cripple Porgy, Michael J. Smartt sets a new standard in the role. He wheels himself around on a rolling cart (the only Porgy in the cast to do it), which heightens the impact of his performance. He sings well enough, he projects strength and tenderness, he reacts to every little thing going on around him, giving us an unusual impact as Porgy. C. William Harwood conducts with vitality, precision, and drama. His associate, Mr. Miner, tends to emphasize the more frankly operatic aspects of the score.

The rest of the large cast and ensemble make us feel we are really living in Catfish Row. George Faison's choreography is at once tight, efficient, and natural. Unquestionably, Radio City is a large space to fill, but the amplification system leaves a few things to be desired. The orchestra in particular sounded tinny and distant at both performances. The vocal miking is better, though enunciation throughout the casts is something less than pristine.