Back in the days when Bogart was a star and not a classic, an evening at the movies meant a cartoon and a couple of short features along with the main attraction or double feature. But in the past quarter century the "short" has all but vanished from the bills of commercial theaters in this country, a victim of television and economics. Now, thanks to a new project of the National Endowment for the Arts, the short could be making a comeback.

Yesterday, the endowment announced that its pilot program, the "Short Film Showcase" which has been distributing a selection of four contemporary short films to theaters nationwide since February, will expand to provide 10 new selections to over 2,000 theaters.

The announcement was made by the endowment's chairman, Livingston L. Biddle Jr. following a screening of the four films initially chosen. These four, including one Academy Award-winner, have so far played to over a million people as preludes to such big name features as "The Goodbye Girl" and "Saturday Night Fever," and audience response has been enthusiastic.

Catherine Wyler, daughter of director William Wyler and assistant director of the endowment's meadia arts program, said, "Apart from the enormous exposure the showcase gives to individual filmmakers, we feel that it will build a vast new audience for independent work."

Exposure has been a problem for producers of short films since the early '50s, when newsreels were replaced by Walter Cronkite, and the Spanky and Our Gang comedies with "The Brady Bunch." The shorts had been dying a slow economic death since the advent of the first full length features. Though the public never lost its taste for such comedy classics as Robert Benchley's "Sex Life of the Polyp," increasing production costs made shorts unprofitable for the big studios. When a government antitrust decision broke up the studio owned chains of theaters, a guaranteed outlet for the outlet for the shorts was lost, and the commercially produced short feature was laid to rest for good entombed in film libraries, and occasionally haunting the airwaves of late-night television. The commercial field has since been limited mainly to industrial and educational films.

In its current incarnation, the short is primarily a vehicle for young, innovative independent artists who cannot manage the expense of a full-length production, and who regard the six-to-10-minute piece a useful medium for experimentation and for the development of cinematic technique.

The four films screened here yesterday gave some evidence of this potential. In "Clay," the work of 35-year-old Washington native Eliot Noyes, clay animals wriggle, stretch, divide and swallow themselves and each other, live, - all to an upbeat score of cool jazz. Noyes, who made the film in 1964 while still in college, observed yesterday that the short-time maker faces an uphill struggle in a country that delights in bigness.

"People who want to do things on a small scale get squashed," he said. But shorts provide the opportunity to experiment through a wider range and with less at stake, than a full-length work. "And that's why the audiences like them. And that's why I like to make them. You get tied up in knots making a long film. . . ."

"Lapis," by James Whitney, presents an evolving pattern of swarming, gyrating dashes of color against a background of uptempo sitar music. These images, which were produced with the use of a computer, resemble those of a kaleidoscope, psychedelically employed.

"Frank" is a wry piece of autobiography, employing multiple soundtracks, like levels of consciousness. At one point, the subject recalls thinking while in college that he was destined to become a great architect. "I thought that was what I was meant to do." It wasn't. The film won an Oscar.