Michael S. Dukakis needs to make an impression. George Bush just needs to avoid making a mistake.

In an election in which a largely unimpressed electorate has swung back and forth and back again between the two presidential candidates, both Democratic and Republican analysts agree, Sunday night's face-to-face encounter probably represents Dukakis' best chance to regain the momentum in a race that seems to be slipping away to Bush."I think the first debate will be the critical moment in the 1988 presidential election," said William Galston, a public affairs professor at the University of Maryland. "Right now, there is a strong political flow in Bush's direction. If the basic presumptions of the electorate are not changed, if they just vote on the basis of what they know and feel now, then in all probability George Bush will win the election."

For 90 minutes, many American voters will "tune in" to the election, some of them for the first time, he said. And after the debate, "I think a lot of them are going to tune out" again.

So while the vice president can be satisfied with a draw, the Massachusetts governor needs to do better than that, establishing his own distinctive vision for the future and his ability to take the country there and, ideally, pushing his opponent into making some damaging gaffe.

"The thing to keep in mind," GOP television consultant John Deardourff said, "is that this is the first time that voters will have seen these two together anywhere and the audience will be extremely high, 75 million or more. It's the first time where together they will be facing the American voters with none of the typical filter or screen that would normally be imposed by the news departments of the television stations. This is television as a funnel, rather than as a filter."

"Americans will watch the debate as a way of reading `Cliffs Notes' for the campaign," Republican consultant Tom Korologos added. "They'll think, `Gee, I'm informed; I watched the debate,' which means this is quite important."

For weeks, each side has been touting the other's prowess in an attempt to reduce expectations for their own man; in fact, neither side can take all that much comfort from their candidates' performances in debates during the primaries. Both were judged adequate but not outstanding, acting like traditional front-runners as they tried to stay above the fray while others attacked.

Dukakis has participated in 21 debates, four of them one-on-one; Bush in seven debates, one of them one-on-one. Dukakis' record as a debater in gubernatorial contests is mixed,as is Bush's record in the vice presidential debates in 1980 and 1984.

Sunday night's nationally televised debate, at Wake Forest University in Winston-Salem, N.C., will have a tightly controlled format, with a panel of three journalists directing questions at a candidate, who will have two minutes to answer.

But the journalists won't be able to follow up their questions. Nor will the candidates be allowed to attack each other directly. Observes Simon Hoggart of the London Observer Service: "It should be like watching a boxing match between two men in straitjackets."

Both candidates are getting advice from all quarters on what to do.

Dukakis is widely urged to concentrate on two different, and potentially conflicting, goals: Get tough to unnerve Bush and to assert strength, your "masculinity quotient with middle America," in the words of political author Kevin Phillips. At the same time, key Democrats and others advise: look more human, emotional, compassionate _ the way you did in your acceptance speech at the national convention in July _ so viewers can "connect" and feel comfortable.

Dukakis' press secretary, Dayton Duncan, described the debate as an "opportunity for the voters to see Dukakis, hear directly from him and be able to judge for themselves who he is and what he stands for."

Norman Ornstein of the American Enterprise Institute agreed. "He has to come across as a reasonable, centrist, knowledgeable, firm and strong individual," Ornstein said, "not some crazy, wild-eyed liberal whose favorite leisure activity at night is to go home, take off his shoes and burn a flag."

And Bush, Ornstein went on, needs to avoid "making a stupid mistake that reinforces some of the doubts about him." Bush is expected to be thoroughly scripted, including some one-liners drafted by media adviser Roger Ailes.

Bush's own aides list avoiding mistakes as a top priority. "He wants to avoid costly mistakes, remain on the offensive as much as possible, stay with our message and appear presidential and in command," said Richard Bond, Bush's national political director.

In 1976, it was President Ford's bewildering assertion that Poland was not under Soviet domination that delivered the debate to challenger Jimmy Carter, who won the election in a squeaker.

In 1980, it was challenger Ronald Reagan's reassuring presence and aw-shucks affability that undercut President Carter's depiction of his opponent as a dangerous extremist. Carter had been leading in many polls before the debate; Reagan won in a landslide.

In 1984, it was President Reagan's stumbling, bumbling manner in the first debate that gave challenger Walter Mondale his one and only shot at being competitive in the race, though Reagan recovered in the second debate and easily won re-election.

As that record shows, debates are more likely to help the challenger than the incumbent, which was one reason Dukakis' strategists would have preferred more debates, while handlers of Bush, the quasi-incumbent, favored none at all. Perhaps more important, any debate gives the challenger equal stature with a more experienced opponent, a situation that at least temporarily would rob the vice president of his single most valuable campaign asset.

The Bush team settled for agreeing to debate the number of times now considered de rigueur in presidential contests _ twice for the presidential nominees, once for the vice-presidential nominees _ and maneuvering the schedule so the debates took place in the midst of big-time sporting events including the Olympics in hopes of minimizing the attention they would get. The second presidential debate will be Oct. 13 or 14, depending on the baseball playoffs.