Sergio Leone's "Once Upon a Time in America," now at area theaters, can serve as a testimonial to persistence: If memory serves, Leone first proposed branching off from epic western reveries, his sub-genre of distinction, into an epic reverie about gangsters back in 1968.

But this would-be epic schlep, dragging almost 50 years of chronology over a sluggish 140 minutes, is far too slight of text and ponderous of presentation to sustain more than nodding-off dramatic interest.

According to reports, Leone structured the scenario in an elaborately looping flashback pattern that was then discarded in favor of a straight chronology, which introduces the principal characters as delinquent kids on New York's Lower East Side in 1921 and plods ahead. It dwells on their careers as organized criminals during the Prohibition period before wrapping up an enigmatic ghost story motif with an epilogue set in the late '60s. Since the present running time diminishes Leone's preferred "final cut" by more than an hour, it's obvious that a number of characters and straggling plot threads have been left on the cutting room floor, probably destined to be retrieved, for those who imagine they've been cheated of a masterpiece, on video editions.

The juvenile chapter seems far too long for the movie's good, since it delays the entrance of the leading players, Robert De Niro and James Woods, for about 45 minutes. The compensations are supposed to be savory period recreation and character exploration, with Leone establishing gang loyalties in boyhood friendships and setting up a lifelong romantic obsession to haunt the De Niro character, "Noodles" Aaronson.

This moniker is a clue to the movie's ultimate lack of dramatic authority. There's something about the nickname "Noodles" that rings preposterous in the context of a solemn chronicle of criminal endeavor. Moreover, it doesn't suit De Niro, who actually enters affecting such a yokelly appearance that you'd feel more comfortable calling him Rube or Zeke. This is nothing compared to the disbelief encouraged by Leone's insistence upon identifying his gangster cronies as Jewish-American, despite the hilarious inability of cast and director to impose a Jewish flavor on their personalities or social setting. Indeed, to the extent that the characters suggest any ethnic dimension, they resemble the sort of incorrigibly adolescent Italian-American hotheads and slobs one associates with the movies of Martin Scorsese.

Since De Niro has served as Scorsese's principal acting instrument for better than a decade, Leone's delusion about doing a story of Jewish-American hoods seems to defy all logic and common sense. The movie itself provides overwhelming evidence of his failure to put across this fond misconception. When Joe Pesci, who played De Niro's brother in "Raging Bull," enters for a brief, entertaining cameo (assisted by Burt Young, whose presence also perks up the proceedings for a scene or two), Leone has a virtual Scorsese reunion on his hands. But for some reason he refuses to recognize that he'd also be on more familiar terms with Italian-American characters. Weird.

The gang, organized by Woods, an ambitious, manipulative type contrasted rather gauchely with De Niro's pensive Noodles, never seems plausibly engaged in any line of criminal activity. Based strictly on the movie's documentation, their predatory skills appear confined to sexual molestation, and Tuesday Weld has a conspicuously degrading role as a Temple Drake type who responds so enthusiastically to assault that she turns pro.

Elizabeth McGovern, looking more baby-faced than ever and affixing a strangely Irish lilt to her lines as a Jewish tantalizer named Deborah, appears as the source of Noodles' romantic longing. The cuts evidently eliminated an ironic kicker in which her character reappeared during the epilogue to inflict a parting disillusion on the elderly Noodles. Despite the silliness of his identity and the lumbering pointlessness of the narrative, De Niro contrives to ingratiate himself as the movie trudges along, evolving into an island of gentlemanly dignity in a sea of grotesques. He even looks wonderful when made up to resemble an elderly gent.

While Leone brought a distinctively spacious, dreamy sensibility to westerns, his mood of contemplation was always vaguely freakish and preposterous as well. Gangster stories seem to belong to the wrong sort of landscape for his picturesque eye and dilatory storytelling sense. Perhaps if he'd envisioned the major characters as immigrants or city toughs transposed to the American West in the twilight of the frontier period the conception would have given him room to fantasize and embroider effectively in both genres.

As it stands, "Once Upon a Time in America" seems stale and antiquated, the dogged but uninspired realization of a dubious filmmaking dream. "The Good, the Bad and the Ugly," "Once Upon a Time in the West" and "Duck, You Sucker!" remain the definitive, inimitable examples of Sergio Leone in an inspired, expansive state of cinematic self-hypnosis.