ON HIS NEW album, "Tougher Than Leather," Willie Nelson plays the part of an aging gunslinger looking death squarely in the eye. With the steady, somber tone of a dying man facing up to his fate, Nelson plucks his unaccompanied acoustic guitar and slowly drawls, "Was it something I did, Lord, a lifetime ago? Am I just now repaying a debt that I owe?" After a telling pause the implies a yes to both questions, Nelson muses, "Justice, sweet justice, you travel so slow," then stubbornly insists, "but you can't change my love for the rose."

These themes of death, faith and justice -- so succinctly stated in this opening sone, "My Love for the Rose" -- are developed at length through the album's 13 interconnected tracks. Nelson, who will be 50 next month, was reportedly inspired to write this album when his lung collapsed in Hawaii in the summer of 1981. During a long convalescence he wove his own encounters with mortality into the mythology of the Old West. The result is "Tougher Than Leather" (Columbia QC 38248), the most ambitious and successful work of an already accomplished career. Nelson will perform selections from the album at the Capital Centre tomorrow night.

After 16 years as a successful country songwriter, Nelson finally broke through as a singer with the 1975 album "Red Headed Stranger." In the eight years since then, however, he has largely neglected his songwriting to concentrate on albums of pop standards, duets, tributes, gospel hymns, soundtrack scores, Christmas songs and live shows. So it is welcome news that "Tougher Than Leather" features nine new original songs.

Like those in "Red Headed Stranger," these songs are arranged to form a narrative. Unlike that earlier celebration of rugged individualism, this new collection of songs is a somber mediation on death and love. As such, it lacks the rousing punch and accessible sentiments of Nelson's more popular songs. Most of the new songs proceed at a deliberate pace, but they gradually draw the listener in by focusing on hard questions with no easy answers.

The opening song depicts death as life's inescapable sentence. Yet in the next song, "Changing Skies," Nelson uses the metaphor of a bird flying through gathering clouds to describe love as another constant. Just the same, he points out that love provides no freedom from the indifferent, ever-changing weather.

These first-person statements are followed by the title song, the tale of an aging gunfighter who shoots down a young challenger and then cruelly crumbles the rose of the dead man's sweetheart. Old Tougher Than Leather (as Nelson calls him) rides off into the sunset but is haunted by the memory of the young woman and the kind of love he has long denied himself. Before he can find her, however, death hunts him down and delivers its own brand of justice.

The mirror reversal of this tale is "Somewhere in Texas (Part 2)." In this version, death's justice is an amoral as it is inexorable. A young cowboy is arrested for a fatal stick-up committed by someone who looks like him. Sentenced to die in the electric chair, he is sustained by the love of his girlfriend and sits with her rose in his cell. He dies just the same, though, for as Nelson puts it in the closing song, "Nobody Slides, My Friend."

We all live under a death sentence, Nelson implies, and there are no pardons. Love cannot provide freedom from that sentence but can offer some meaning in the face of it. In "Summer of Roses," Nelson faces up to this fact with a wistful, longing vocal over Johnny Gimble's slow fiddle solo. "A short time I have to be with you, my love," Nelson acknowledges, "but a short time is better than no time at all."

The album's only up-tempo tune is the first single, "Little Olf Fashioned Karma," which sports a brisk, crisp Western swing. Fiddler Gimble, pianist Bobbie Nelson, guitarist Grady Martin and harmonica player Mickey Raphael all get off smart solos. Several numbers have a midtempo clip-clop, but most are slow, melancholy ballads with very austere arrangements, Nelson's gravelly voice and spare, eloquent guitar picking usually dominating.

Fortunately, Nelson's vocal phrasing is so finely honed that it can carry this responsibility. When Nelson sings as a condemned man, he offers his love "the dry leaves of autumn to soften the fall of your snowflakes." The words slip from his pauses so reluctantly that he clearly acknowledges the shadow of mortality lurking behind his affection. This is a landmark album.