Jack Nitzsche's main theme for "Blue Collar" stresses a heavy-footed, echoing bass phrase that suggests the approach of something ominous and inexorable: maybe the abominable snowman, who seems to ascend the stairs in four ponderous giant steps and pound twice on the door with a big muffled paw.

"Blue Collar," opening today at area theaters, is not a monster melodrama, but probably without meaning to, Nitzche sure got the number of the director he was working for - 31-year-old Paul Schrader, the screenwriter of "Taxi Driver," making his directing debut. Ambition and gloom hover over Schrader like a storm cloud; even in casual conversation he tends to impose a brooding, tormented mood.

"Blue Collar" is a lugubriously serious approach at proletarian tragedy. Schrader and his older brother Leonard collaborated on the screenplay, an often expendient, yet doggedly involving, chronicle of the decline and fall of three friends who work on the assembly line at an auto plant. They get in over their heads when they try to get ahead of a few nagging debts and resentments by breaking into the vault at their union local.

The larcenous workers, two worried family men played by Richard Pryor and Harvey Keitel and a brawny, easygoing former convict played by Yaphet Kotto, find only nominal amount of cash.However, they decide to blackmail the local union boss, Harry Bellaver, by threatening to publish an incriminating ledger documenting mob loans. Inevitably , the scheme fails. A factory accident is arranged to eliminate Kotto, while Pryor and Keitel end up at each other's throats, one having made a deal with the corrupt union leadership and the other having made a deal with the police.

Schrader evidently hopes audiences will trudge off murmuring, "Heavy, heavy, heavy." Some may, because there's enduring social validity in his cliched assertons that you can't beat the system and that you guarantee only grief for yourself if you try. Schrader might make his points more effectively if his style weren't go heavy, heavy, heavy. "Blue Collar" is an excessively schematic and intellectualized demonstration that the odds are stacked against the wageearning underdog.

Martin Scorsese had the filmmaking technique and volatility that could release the emotional storm Schrader brewed up when he wrote. "Taxi Driver." As a director, Schrader is too stiff to vary the brooding tension that comes naturally to him, or trigger revelatory climaxes and confrontations.

A snoopy FBI agent played by Cliff De Young is disparaged for trying to pry information out of the men about their union, but Schrader is no less presumptuons. "Blue Collar" always feels like the work of an artistic outsider and interloper, a theorist imposing a view on his subjects rather than placing himself imaginatively inside their skins.

This limitation may account for the fact the Keitel's white worker never seems to harmonize with the blacks embodied and individualized far more believably by Pryor and Kotto, although the three are supposed to be bosom buddies. The most incongruous detail in the film appears to be Keitel's presence at a weekend sex party Kotto gives at his apartment.

Kotto projects the most sympathetic personality, but Pryor gives the most intriguing performance, since he's trying to extend his range. There's a trap awaiting viewers who overreact to his character's crowd-pleasing displays of defiance and sarcasm. Underneath it all his character is a sellout. Pryor risks a certain degree of popular disillusion by portraying someone whose insolence is a mask for corrupt self-interest, but it's a worthwhile, compelling risk.

Like the Dustin Hoffman film "Straight Time," Schrader's picture sustains a certain interest despite its faults. You stick with both movies because of the promise of something authentic and tragically revealing, even though the promise is never fulfilled. These films don't really work, but they're the sort of films that don't work in interesting ways.