Showdown at TV corral. Much hangs on the outcome of the candidates' encounter Sunday night. Dukakis hopes to recharge his campaign, Bush to show he's `one of the guys.'

The first presidential debate on Sunday night looms as the most important event of the 1988 fall campaign. When Michael Dukakis and George Bush step before live cameras for 90 minutes at 8 p.m., Eastern time, millions of Americans will get their first good look at these men under fire.

For Governor Dukakis, trailing in the latest nationwide Times Mirror poll by six points, the debate presents an invaluable opportunity. With a single stroke, like John Kennedy in 1960, he has the chance to turn this race around.

For Vice-President Bush, whose stock has soared since the Republican National Convention, the debate offers an occasion to reach out to independents and disaffected Democrats, and to show he's just ``one of the guys.''

Each candidate has spent days getting ready for this showdown, which will take place at Wake Forest University in Winston-Salem, N.C. (Wake Forest gets ready, Page 4.)

The two men have tried to anticipate most of the 20 to 25 questions that will be asked by a panel of journalists and to frame their answers in advance.

Though this is called a debate, it really isn't. As Richard Nixon learned to his chagrin in 1960, it is not debating points that count. This is really a chance for these two men to communicate directly with voters, to make contact with them through the television lens, to demonstrate their personalities.

Mr. Dukakis, a practiced TV moderator with his work on ``The Advocates'' program in the early '70s, has more experience on camera. But Thomas Mann of the Brookings Institution says Dukakis could be hurt by a lack of warmth and humor, both essential ingredients of Kennedy's winning performance 28 years ago.

Mr. Bush, less practiced on television, nevertheless did well in his primary debates with Republican opponents. But his often scrambled syntax and sometimes stern demeanor are not ideal for TV.

Both candidates will be under great pressure. But Dukakis's task will be more difficult, for he is less known, and his political message is more complex.

The candidates need to do three principal things:

First, avoid major mistakes. A single gaffe - as President Gerald Ford learned in 1976 - can stagger a campaign. When Mr. Ford implied that Eastern Europe was not under Soviet domination, it raised questions about his basic competence in foreign affairs.

Second, look presidential. As Bob Dole learned in 1976 and '88, anger and meanness almost instantly turn off voters. The candidates need to appear tough, but in control. Sweating and looking nervous also hurt, as Mr. Nixon found in 1960.

Third, correct their negatives. This is imperative.

Dukakis has skidded in the polls, primarily because of three unfavorable impressions of him that have been fostered by Bush.

The most serious of these is the perception that he is a Massachusetts liberal who is soft on criminals. Bush has hammered at Dukakis's support of a prison furlough program. Under that program, first-degree murderers with no chance of parole were permitted to go on weekend passes. One of them, Willie Horton, went on a violent rampage.

Dukakis is also painted as weak on defense, a man who opposed ``every weapons system since the slingshot,'' in Bush's words. That charge has hurt Dukakis with traditional Democrats, who remember that no one was tougher on defense than Democrat Harry Truman.

Dukakis must also deal with the pledge of allegiance issue. Bush has denounced Dukakis's decision to veto a bill that would have required all Massachusetts teachers to lead their classes in the pledge. The governor, who says the bill was unconstitutional, angrily charges that Bush is questioning his patriotism.

Andrew Kohut, president of the Gallup Organization, says Dukakis is missing the point. Gallup studies show that voters consider the veto an indication that Dukakis is ``wrongheaded'' about a fundamental issue. The major impact of the pledge issue was to help Bush portray Dukakis as an out-of-step politician.

Bush has serious problems of his own, however.

In the policy area, Bush still suffers from the Iran-contra affair, and the implication that he approved selling arms to Iran. It undercuts his claim of foreign policy experience.

Even more harmful are charges that the White House was dealing through the back door with the drug-running strong man of Panama, Gen. Manuel Antonio Noriega. Fresh fuel was thrown on that fire yesterday, when a former Noriega aide claimed that Bush knew of General Noriega's money-laundering five years ago - something Bush has denied.

Bush will also be trying to show he understands the problems of average Americans. That won't be easy. Bush looks very comfortable in his government limousine. He is an ``establishment'' Republican, closely tied to Washington insiders. To overcome that image, he must demonstrate greater humanity, political experts say.