This Wagnerian tale of art and politics is more thesis than theater; The Music Keeper Play by Elliot Tiber and Andre Ernotte. Starring Jan Miner. Directed by Mr. Ernotte.

In their fictional drama about Winifred Wagner, Elliot Tiber and Andre Ernotte attempt considerably more than they achieve. ''The Music Keeper,'' at the South Street Theater, seeks to explore nothing less than Frau Wagner's relationship with Adolf Hitler and her obsession with the Bayreuth mystique and Hitler's malign mission.

According to a program note, the authors' research into ''the roots of the Wagnerian power . . . uncovered what, they felt, was the most phenomenal and gruesome marriage of art and politics of the century.''

''The Music Keeper'' begins with the arrival in Bayreuth of a young American conductor named Andrew (Dennis Bacigalupi). The time is March 3, 1980, two days before Frau Wagner's death. Andrew has come ostensibly to interview, but perhaps to murder, the formidable widow (Jan Miner) of Richard Wagner's son, Siegfried. As the docudrama moves back and forth in time, Mr. Bacigalupi plays a number of roles, including the prosecutor and defense lawyer at Frau Wagner's de-Nazification trial. (The court sentenced her to 450 days of special labor as a Nazi collaborator.)

In a series of dialogues and interrogations, the authors expose the fraudulence of Frau Wagner's self-serving protestations that she was apolitical, that there was nothing sinister in her close relationship with Hitler, that she succeeded in saving some Jewish lives, and that her sole mission was to preserve the Wagnerian heritage against all adversities.

The trouble with this factually based stage fiction is that it is more thesis than theatrical, and that it tends to confuse rather than clarify. The statuesque Miss Miner gives a commanding, believably accented performance as the arrogant octogenarian, a sometimes subdued but never submissive Brunhilde.

Frau Wagner is finally reduced to screaming incoherence when her enigmatic visitor follows a succession of Wagnerian excerpts with a recording of Franz Lehar's ''The Merry Widow,'' which, he insists, represents Hitler's real musical tastes. Mr. Bacigalupi struggles manfully with his baffling assignment, but he is defeated both by the writing and by Mr. Ernotte's agitated direction.

For the scene of the action, Karen Schulz has devised an atmospheric impression of the salon in the Richard Wagner-Wahnfried House, with shadowy blowups of figures from Wagnerian and Hitlerian mythology. Susan Tsu costumed the production, Rachel Budin lighted it, and Mildred Kayden served as adviser for a musical program that includes Nazi storm trooper songs along with Wagneriana.