You can take the boy out of Hollywood.

But.

Ronald Reagan is going to debate Walter Mondale on television Sunday night even though some of Reagan's own advisers have cautioned him against it. An enormously popular incumbent has little to gain and potentially much to lose by agreeing to debate, but Reagan wants to debate anyway.

Why? Partly for the obvious reason that it looks bad to duck a fight. Not manly. But also for this reason: once an actor, always an actor. Ronald Reagan loves to perform. He loves to play the role of president of the United States. He loves being a hit. And he knows this will be his last chance to play the part of the beloved incumbent warding off the impertinent interloper. He can never run for president again.

Would an actor play Hamlet and skip the dueling scenes? Ronald Reagan can lose the debates and still win the election. But Walter Mondale cannot win the election without winning the debates -- a reminder of how important television and the debates are in this campaign. In effect, Reagan is giving Mondale his only chance to win. Reagan has been told that the negative feedback he would suffer from refusing to debate is much less potentially damaging than a bad Reagan debate performance could be, but he won't listen.

Those who tell him that are forgetting that Ronald Reagan does not consider himself capable of a bad performance. Not in the saga in which he now stars. Actors can get this way when they're on a roll; they get cocky and drunk with approval. Reagan has developed supreme (and justifiable) confidence in his own bag of tricks, in the way he portrays himself on TV. It is a masterful characterization, a blend of Grandpa Walton, Douglas MacArthur and George Gipp, with a little folksy touch of Fred MacMurray in "The Absent-Minded Professor" thrown in. Reagan has figured out that when he is answering a question or making a statement and does that little bit where he shakes his head as if rattling loose a thought and then says something like "Well, I just think . . . ," people are going to find this endearing and cute. Actors and politicians are alike in that both see everything they say and do in terms of desired effect, of winning people over. Applause or votes -- same thing, really. Reagan was handed scripts at Warner Bros., he is handed speeches now. He clearly loves it when one of his jokes gets yocks, and just as clearly relishes the chance to make folks puddle over with one of his famous heart-tugging patriotic bromides.

And if he screws up, well, they can always save it in the editing.

Although not with a debate. Even with a debate as stuffily formatted as the creaky League of Women Voters setup, Reagan can't be saved by editing. He'll have to recoup, if necessary, with twinkling, or shucks'ing, or with his get-tough routine. People watch these debates wondering which candidate will make a goof, not which will make the best impression.

Yet the worst goof Reagan could make would have to be phenomenally terrible to hurt him. A little goof, one that narrowed the gap between him and Mondale, Reagan wouldn't mind. He wouldn't mind it because he's very likely bored with the lopsided advantage he enjoys now. It isn't fun. It isn't a fight. It's not good entertainment. In 1980, when things were neck and neck for a while, Reagan savored the struggle. It was of movie-epic status. He doesn't want to sully his image of niceness by appearing to be a bully, and underdogs can become suddenly very attractive. Reagan's acceptance speech at the Republican convention was disproportionately combative and bellicose, probably because Reagan wanted to pretend this really was a battle royal.

Reagan's emergence from the cocoon to debate Mondale on live TV is the end of his phantom candidacy, but just another Act 2, Scene 3 for Reagan. The debate is the real start of the campaign, because most people haven't been terribly caught up by all the hullabaloo up to this time, hard as it may be for political junkies in Washington to believe. One Capitol Hill veteran points out that opinion polls tend to freeze the minute a debate is announced, because no minds are likely to be changed by anything now but the debates themselves.

Thus the real politics begin: The telepolitics. Telepolitik. All the other stuff was just warm-up, diversion for a relatively small minority.

Ronald Reagan is a ham. It isn't being disrespectful to say so. Many other presidents, many other politicians, have been probably just as hammy. Anyone can see in newsreels the enormous pleasure FDR took in melodramatics and in pitching a corker of a joke right over the plate. Teddy Roosevelt appears to have had a high time of it. John F. Kennedy relished the challenge of the live press conference and the opportunity to demonstrate his quick wit. Reagan is an actor first and a politician second, so live press conferences are not what he likes best. He is at his best all alone with a script, a couple TelePrompTers and a camera, and 50 million people peering at him from their homes.

Reagan's advisers prefer he stick to scripts. When he is left on his own to ad-lib, he can come up with embarrassments like the kitchen-remodeling analogy for tragic bombings in Beirut. They would prefer he not debate.

Incumbents have more to lose in debates, and traditionally have lost it. Gerald Ford freed Eastern Europe from communist domination in a 1976 debate with Jimmy Carter. That single mistake was what most people took away from the debate. Then Carter looked duncey when he brought up daughter Amy's nuclear nightmares in the 1980 debate with Reagan. It may have contributed heavily to his defeat, largely because it supported widespread misgivings about him. Reagan knows the precedents and probably wants to be the one to break the pattern.

And he will do it with sheer showmanship, the thing he knows best. Walter Mondale could be the greatest debater in the world, but debating skills are practically irrelevant with Reagan as the opponent. It's not going to be Marquess of Queensbury. Reagan rewrote the rule book.

Reagan won the 1980 debates merely by looking presidential and by coming up with one crucial catch phrase, the old are-you-now-or-have-you-ever-been better off routine (to paraphrase). Thus the most memorable part of the debate, other than Carter's misfire, was not a spontaneous moment at all, but one that Reagan had planned and rehearsed. Undoubtedly he will come up with some new nifty this year, though he presumably won't have George Will to coach him. Will now holds forth on ABC News, delivering the Reagan line disguised as commentary on "World News Tonight" and on "This Week With David Brinkley."

As for looking presidential, that is now no worry for Reagan. Whooo, no! He has helped redefine what looking presidential is. We will expect presidents to look and act like him for years. Actors cast in the role of presidents in movies and TV shows will probably fall back, consciously or not, on Reaganny mannerisms and postures. The advisers worrying about a Reagan gaffe may be worrying for nothing. It seems the American viewer-voter will forgive Reagan any gaffe and any goof because he looks so good in the president costume. Reagan has updated tell-them-what-they-want-to-hear to be-for-them-what-they-want-you-to-be.

He is, to use a currently popular sensitivity-cliche', there for us.

All actors have large egos. They adore the attention, they adore the adoration. Especially movie actors, who are never quite certain if it's them or the way the movie was put together; in this, Ronald Reagan is that rarity, an actor auteur, like John Cassavetes (well, sort of) or Orson Welles. Actors love to develop specialties that an audience likes, and when they discover them, they repeat them over and over to get the Pavlovian response from the audience and the Pavlovian response, ego gratification, from themselves. An actor with a sense of mission is particularly driven. Ronald Reagan is not the best actor ever to come out of Hollywood, but he is the best actor ever to occupy the White House, at least since the almighty camera has been around.

Walter Mondale is boning up on "the issues." How do we define "the issues"? Easy. "The issues" are what everyone claims to want to hear discussed, but no one really cares about. Television is not a content medium. It is a form medium. Ronald Reagan is in good form when he is on television. Walter Mondale is in good form when he is shouting back at hecklers. Not bloody likely to happen on Sunday night, unless Reagan decides to do his own heckling for a change.

This will, after all, be less a debate than a monumental installment of "Star Search," except that instead of two unknowns competing for a prize, it will be one Communicateur Grand versus one aspiring pretender. What viewers will be watching for, if past debates are any guide, is not which man makes the strongest impression and the better president, but rather which man's performance is more likely to wind up on a future edition of "Foul Ups, Bleeps and Blunders."

If Reagan makes a major bleep or a blunder, it will be on his own head, and it remains to be seen how durable the Teflon armor is. Reagan detractors marvel at his ability to duck blame and absorb credit. He has been compared to Jack Benny on the old "Jack Benny Show," a man playing a cherished icon who is surrounded with a chorus of stooges and colorful characters who play the fool -- except that Benny made himself a stooge as well. No, Reagan is probably more like Johnny Carson. He exults in the laughs when they come and blames the writers when they don't.

Reagan escapes blame for foreign policy disasters, it has been pointed out, but takes all the bows for the apparent success of his administration's economic policies. No economic advisers are trotted out for kudos. All credit goes to Reagan. Carson has never invited his writers out from backstage to take bows, either. Like Reagan, Carson is able to give the impression of spontaneity when he is following a script; he even has scripted "saves" for jokes that go bad. The camera that stays on Carson all night, so that the director can cut to him for reaction shots, is called a slave camera. The camera is Reagan's slave, too.

Even those who have no complaint of any kind whatsoever with the Reagan administration may stop and wonder sometimes if it's all real, if it's really happening, or if they're part of a movie, too. Reagan says he's president. We believe he's president. He is president. Or at least giving a darn good performance. If the average American television set is on more than seven hours a day as has been claimed, then the average American household is spending half its waking time exposed to an incessant source of fantasy and illusion, an alternate universe populated by electronically generated apparitions. The vicarious is on almost equal footing with the actual; to seem is virtually to be. Ronald Reagan may indeed be the perfect president for this America, 1984.