ONE SPRING DAY in 1953 a reporter in Los Angeles got the hot tip that a nonpartisan citizen's league was pushing one of Hollywood's best-known leading men to run for the Senate. Tracked down on a Universal Studios sound stage, where he was romancing Dorothy Malone in the low-budget western "Law and Order," the leading man in question brushed aside the idea with an easy smile and a friendly shrug familiar to millions of cinema fans.

Politics? Not interested, Ronald Reagan said. "I'd like to keep on making horse operas," he explained " . . .I'm a ham -- always was and always will be."

Over the next two decades, as we all know, Reagan did get interested in politics, trading in his saddle for a soapbox and a career that may well lead to the White House. But in one respect his self-assessment 27 years ago has proved correct: Reagan the politician, like Reagan the actor, is still the same performer -- "ham," if you must -- he always was.

To anyone who has seen candidate Reagan on the stump or on television this year, a retrospective look at the movies he made during his 27 years i Hollywood offers a strange sort of reverse deja vu . The man never changes. Age has wrought some wrinkles, but otherwise today's would-be president has the same broad shoulders, the same happy smile, the same sturdy, authoritative voice, even the same dark brown pompadour wave that marked Andy McLeod, the earnest young radio newsman Reagan portrayed 43 years ago in his first movie, "Love Is on the Air."

Even more eerie, the persoality that Reagan conveys as a candidate -- the engaging, down-to-earth, All-American guy -- is exactly the personality that shines through reel after reel of Reagan's films. And the emotive techniques that made him a star in the '30s and '40s are the same tools that helped make him a winner in 1980.

The halting, tearful tones with which Reagan, as the dying George Gipp ("Knutte Rockne -- All American," 1940) utters his most famous screen line -- "Some day, when things are rough, maybe you can ask the boys to go in there and win just one for the Gipper" -- are the same tones he uses in his stump speech today when he recalls how his father was laid off on Christmas Eve in 1931.

The "aw shucks" shrug and the shy downward glance that Reagan, as General Custer, uses to apologize for insulting Olivia de Havilland ("Santa Fe Trail," 1940) are the same shrug and glance we see on television when candidate Reagan is apologizing for a factual flub.

The pregnant pause and the quick, emphatic nod that add melodrama to Reagan's first love scene with a girlish Jane Wyman ("Brother Rat," 1938) -- "Gee, Claire, you're really . . .swell" -- are the same pause and nod that add feeling to Reagan's current statements about the economy -- "The culprits for inflation are not business and labor. The cause of inflation is . . . government."

The tight-lipped glare and angry voice that Reagan, playing second banana to a chimpanzee ("Bedtime for Bonzo," 1951) employs to discipline his unruly pet are the same glare and voice that Reagan used last February when he blasted a debate moderator who tried to cut him off ("I'm paying for this microphone, Mr. Breen.")

But to say that Reagan is the same man now that he was way back then is merely to repeat the most common theme of the critics who reviewed his films over the years: Whatever role he was assigned, the personality came out the same.

Reagan made 55 feature-length films, mostly for Warner Brothers, between 1937 and 1964; he appeared only as a voice in three of them and as a bit player in a half dozen more.

But in all the rest he had a major role, playing doctors and lawyers, scientists and artists, G-men and gangsters, sheriffs and rustlers, scholars and jocks. He was a reporter, a talk-show host, a migrant fruitpicker, a playboy and a pianist. He had a prolific military career in film, serving as officer and enlisted man in every branch of the armed forces and fighting in every conflict from the Civil War to Korea. As a Secret Service agent, he chased illegal aliens across the Mexican border and crashed a blimp into the sea. But he was always the same old Reagan.

The recurring question -- for film critics then and political analysts now -- is how much of the public Reagan is artifice, how much the real thing. Is the solid-citizen Reagan character an actor's invention, or is what you see exactly what you get?

Judging from Reagan's cheerful autogiography, "Where's the Rest of Me?", a good deal of technique is involved. The candidate explains in his book how hard he had to work as a radio announcer and as an actor to "make reading sound talking." It was a long time, he says, before he perfected what he calls "that easy conversational persuasive sell."

On the other hand, it also seems clear that Warner Brothers hired Reagan to play just what he was -- an unassuming fellow from Main Street, U.S.A. -- and cast him mainly in roles suiting that persona.

When Des Moines sportscaster "Dutch" Reagan was discovered by Warner in 1937, the studio's roster of males stars was thick with tough guys (Humphrey Bogart, James Cagney, Edward G. Robinson), monsters (Boris Karloff), swashbucklers (Errol Flynn), songsters (Dick Powell) and dramatic heavies (Pat O'Brien, Claude Rains). What Warner Brothers needed was a wholesome, likeable youngster who could fill the kind of roles played by Jimmy Stewart at MGM and Fred MacMurray at Paramount.

So the new discovery was cast in dozens of films that might charitably be characterized as harmless fluff. He played the breathless announcer regaling a nationwide radio aduience with reports from the scene of a dazzling Hollywood premiere in "Boy Meets Girl" (1938). He played the straight-arrow who inspires the Dead End Kids in "Angels Wash Their Faces" (1939). He played the undisciplined RAF rake who finally sees the error of his ways and volunteers for a suicide mission over Berlin in "International Squadron" (1941). He played the cavalry private who falls in love with his horse in "Sergeant Murphy" (1938) -- a role first offered to Cagney, who was, Reagan noted later, "smart enought to turn it down."

If candidate Reagan sometimes seems unconvincing when he lashes out at his opponents, he was unsuccessful, too, in most of the films where he was supposed to play a tough guy. When he portrayed mobster capo Jack Browning in the 1964 remake of Hemingway's "The Killers," The New York Times critic observed that Reagan seemed "ill-at-ease" throughout. And in "Nine Lives Are Not Enough" (1941) Reagan just doesn't quite work as the brash, fedora-askew reporter ("On the strength of my story and my story alone, that man's behind bars") who outwits the cops to crack a murder mystery.

As actor and as candidate alike, Reagan has more felicitous results when his role is that of the crusader for decency. In "Hell's Kitchen" (1939), he comes across reasonably well as an advocate of reformatory reform despite the distracting antics of Huntz Hall and Leo Gorcey. And in "Storm Warning" (1950), District Attorney Reagan wows Ginger Rogers and Doris Day with his relentless pursuit of a murder perpetrated by the Ku Klux Klan.

Reagan's personal choice as his best picture is "King's Row" (1942), which was, in fact, nominated for a Best Picture Oscar (it lost out to "Mrs. Miniver"). There Reagan plays a young man who wakes up after an accident to find that both his legs have been amputated. "Where's the rest of me?" the horrified Reagan blurts out, and the line stuck so firmly in his memory that 23 years later he made it the title of his autobiography.

Contrary to legend, "Bedtime for Bonzo," in which Reagan plays a professor trying to raise a chimpanzee like a human child, is not the worst Reagan film. That distinction belongs, by agreement of the star and the critics, to "That Hagen Girl" (1947) -- the only movie in which Reagan plays a politician.

The political ambitions of the small-town lawyer Reagan plays in that film, however, are sublimated to his growing love for Shirley Temple, a high school Lolita whom many townspeople believe to be his illegitimate daughter. The film is so cluttered with cliches that when Reagan went to a sneak preview, the entire audience shouted "Oh, no!" during the big Temple-Reagan love scene. When the screening ended, the embarrassed Reagan "sat huddled in the darkness until I was sure the lobby was empty."

Unfortunately for the body politic, the average television viewer will probably have no chance to see what Reagan was up to during his three decades as a film idol. The Federal Trade Communications Commission has decided that any showing of Reagan's films would trigger the statutory equal-time rule, requiring stations showing the movies to give the same amount of time, free, to all presidential rivals. As a result no stations will show the films.

Rep. Henry Waxman, a California Democrat whose district includes Hollywood -- and who is thus upset about "inequitable treatment" of actor-candidates -- has introduced legislation to overturn the FCC rule. But the bill seems to have little chance of passage this year.

If Congress were moved to act on Waxman's bill, voters around the country could tune in at night and watch what Warner Brothers might have called "The Three Faces of Ronald": as a candidate on the nightly news, as a self-advocate on the political ads, and as an airman or cowboy or baseball star on the late movies.

But they would find rather quickly that the three faces of Reagan all seem to be identical -- that the film star and the candidate are the same man. And then the midnight viewer could contemplate anew that recurring Reagan question: Where does the actor stop and the real person begin?