Soprano Jody Rapport sang a lovely Violetta last night in the Summer Opera Theater Company's splendid new production of Verdi's "La Traviata" at the Hartke Theater. Rapport and Myra Merritt, who was Violetta on opening night, will alternate in the taxing role.

Rapport's performance is a forceful interpretation of the sad and doomed Parisian courtesan. Her finest moments, both dramatically and vocally, come in the wrenching second act scene with her lover Alfredo's father, Giorgio Germont, in which she resists his request to leave Alfredo. There was a sort of Callas-like steel in her defiance. And a bit later, after she has succumbed and is weeping, Rapport's soft phrasing of the passage beginning "Ah, dite alla giovine" ("Oh, say to that gentle maiden") was haunting.

She does not, however, project the heart rending pathos and fragility of Teresa Stratas in the Franco Zeffirelli movie version. In this respect, Stratas' physical tininess is a great advantage in the classic role.

And why, anyway, one might ask, is the company choosing to do this production in competition with the incandescent movie, which is now going into its third month of packed houses here and has just opened at a second theater? The answer is simple, and fundamental.

Not even the combined talents of Stratas, Placido Domingo, James Levine and Zeffirelli can exhaust all the dramatic or musical possibilities of so extraordinary a work. Just to cite one example, the power of those unforgettable Stratas close-ups may be impossible from the Hartke stage, but one occasionally gets a purity of vocal sound from that stage that outdoes the movie's sometimes badly balanced soundtrack.

Rapport repeats her Violetta next Friday.