"The Amen Corner," the new musical that opened a six-week pre-Broadway run last night at Ford's Theatre, depicts the tribulations of a woman preacher in Harlem, whose family is in disarray and whose congregation is about to rebel and take away her pulpit. But the real warfare on the stage is far more subtle. It is the struggle between James Baldwin's play of the same name, which is at the root of this enterprise, and the adaptation that has been fashioned from it by Garry Sherman, Peter Udell and Philip Rose.

Although the musical has a gifted cast and a second-act finale--"Rise Up and Stand Again" that is a real rafter-shaker (or could be if the show didn't have such problems getting there)--Baldwin seems to have one thing on his mind, while his adaptors have another.

Sherman, Udell and Rose are the gentlemen who took Ossie Davis' spunky comedy, "Purlie Victorious," and turned it into the highly successful musical "Purlie." That one, not so coincidentally, happened to be about a dirt-poor Southern black preacher and braggart, who was trying to wrestle his church back from a cantankerous redneck. Spirited as a play, it proved just as spirited as a musical. This time, however, the three men are working with far more serious material. While it may be unjust to say they have "Purlie-ized" it, they are clearly applying dollops of sentiment and comic leavening that have the chief result of lessening the story's impact.

"The Amen Corner," as Baldwin wrote it, is an angry study of Sister Margaret, a woman who has found that the arms of the Lord offer the only safety from the dangers of a racist world and the heartache of an errant husband. For 10 years, she has preached His word with unfailing rigor. But in her concern for saving souls, she often fails to notice human beings--among them, her teen-age son, yearning for a life of his own in the world outside the storefront church. Baldwin intentionally put his heroine in "a hard place." Trying to treat her husband and son as men and save them at the same time, she turned zealously to religion, and religion, in the final count, robbed her of her compassion and brought her down.

Serious stuff, and not the stuff of the usual musical. The creators of the show at Ford's, however, are not about to blaze new trails. Baldwin's story has been followed more or less, but it has also been considerably softened. The characters' rough emotional edges have been sandpapered down and a far more conventional sense of morality made to prevail. Sister Margaret, although played with great sincerity by Rhetta Hughes, is a cold fish, who seemingly just couldn't get along with her jazz musician husband and therefore left him. The perils of their life together, the stomach-churning fears and the dark trauma of having to bury their first child in the cold ground are glossed over, reduced to vague "Dear Abby" notions of incompatability.

In Baldwin's play, the members of her flock have come to resent her unbending righteousness. But in the musical, they have been transformed into the equivalent of wicked step-sisters and their deep hate made into neighborly envy. In a sassy, musical comedy number, "In the Real World," they voice their grievances. A certain crowd-pleaser, the number also has the decided effect of taking the edge off the evening's drama.

In general, the score, highly reminiscent of "Purlie" with its rolling gospel rhythms and not-quite-soulful ballads, tends to intrude on the play, mitigating the real issues and sometimes subverting them entirely. Much of the music is pleasantly toe-tapping, although something more than pleasantness, something gutsier and more impassioned, may be called for. When, for example, the son (the talented Keith Lorenzo Amos), informs his mother that he is bucking her authority and flying the roost, he does so in a perky little melody that seems to suggest he is fleeing in a horse and shay. At Sister Margaret's lowest point--her husband is in the back room dying of TB and the congregation is about to lower the boom--the show serves up a rollicking gospel number, "Leanin' on the Lord," that is really nothing more than a truce in the dramatic proceedings.

What carries the show as far as it goes--which is about halfway--is a cast equipped with splendid musical gifts. These people could bring down the walls of a Greek temple, let alone the flimsy sets by the usually dependable Karl Eigsti. Hughes is an uncertain actress, but when she sings, it is with the clarity of an archangel's trumpet and the conviction of all 12 apostles. As her sister, Ruth Brown makes an ordinary ballad, "Somewhere Close By," ache with feeling. Chuck Cooper, Helena-Joyce Wright and Jean Cheek--the leaders of the rebellion--carp and cluck with great gusto, and it's not their fault if the glee they dispense appears vaguely misplaced. Even Roger Robinson, as Margaret's wayfaring husband, manages to keep a grip on his dignity, although the show deals him a mawkish hand, indeed.

But that seems to be the trouble as a whole. Baldwin's play may not be his best, but it has the courage of its rage. The musical pulls back constantly, pretends to tell it as it is, and then tells us as it is only in the benign land of escapist Broadway musicals. For fear of being downbeat, it is occasionally dead beat. And in its attempts to be heartfelt, it sidesteps the heart of matters with a blitheness that, unfortunately, has less to do with Baldwin than with box office.

THE AMEN CORNER. Music by Garry Sherman; lyrics, Peter Udell; book, Philip Rose and Peter Udell; based on James Baldwin's "The Amen Corner." Directed by Philip Rose; choreography, Al Perryman. With Rhetta Hughes, Roger Robinson, Keith Lorenzo Amos, Ruth Brown, Helena-Joyce Wright, Chuck Cooper, Jean Cheek. At Ford's Theatre through Oct. 23.