In his half-hour campaign film, President Reagan says his wife, Nancy, "has been my First Lady since long before the White House . . . . I can't imagine life without her." It ends with them putting their arms around each other's waists as they walk across a verdant field.

The same film has television clips of Reagan addressing the veterans of the Normandy landings at the emotional 40th anniversary of D-Day last June, which moved even reporters and Democrats to tears.

At the University of Southern California two weeks ago, Walter F. Mondale responded to Reagan's assertion that, when it comes to his presidency, "You ain't seen nothing yet." In shirt-sleeves and with his tie down, Mondale shouted at his audience: "That's right, and when it comes to arms control, we ain't seen nothing at all!"

On another occasion, when asked if the circles under his eyes are the result of campaign fatigue, Mondale responded forcefully, paraphrasing a television commercial: "I got these bags the hard way. I earned them!"

In an age when television is the dominant factor in presidential campaigns, these examples illustrate the difference between Reagan and Mondale in their use of the medium -- and the resulting differences in their images.

Mondale comes off at a sharp disadvantage, appearing shrill, harsh -- sometimes even menacing. Yet, at the same time -- primarily because of his voice -- he comes off as somehow "weak" and a "wimp," something his friends and associates find incredible.

Reagan is never shrill or threatening. He is the narrator of one of his commercials and his strong, confident voice, as smooth and mellow as 20-year-old scotch, soothes and reassures as he extols the accomplishments of his first term: "Americans are working again and so is America . . . . Now it's all coming together. With our beloved nation at peace, we are in the midst of a springtime of hope for America. Greatness lies ahead."

The issue of the candidates' images is much more than a matter of cosmetics. The ability to communicate effectively is an essential element of leadership, and Reagan's dominance in this area, as measured by the opinion polls, has been the most devastating to Mondale's campaign. And it obviously will be a major factor in Sunday's debate, which is crucial to Mondale.

These poll results are exasperating to Mondale and his supporters, because they think the public's perception of the two candidates is wrong.

"Mondale's not a wimp," one Capitol Hill Democrat said last week. "He's self-confident, a man's man, a towel-snapper in the locker room who likes to smoke cigars and drink and talk politics with the boys. But something clicks in him when the television lights go on. He's not natural.

"Reagan comes across a little goody-goody, a little prissy and his real personal toughness doesn't come through. He's got a temper, and he doesn't like people who criticize him or disagree with him. Look at that guy Gary Richard Arnold who was a House candidate and was badgering him at the White House a couple of years ago until Reagan yelled at him to shut up. That guy stripped it away."

Part of Mondale's image problem is having as an opponent an unsurpassed master -- by experience and training -- of television and manipulation of the media. Part is his reserve, which prevents voters from feeling they're getting to know and trust him, and the fact that he campaigns in terms of programs and specific issues, rather than tradition and values.

Part of the problem is his voice and delivery, which are exacerbated by the sort of events he has often scheduled. And another part is simply his circumstance of challenging a popular incumbent in relatively good times.

"I don't think anyone would do very well against Reagan right now," one Democratic campaign manager said. "He's the 800-pound gorilla with peace and prosperity."

Whereas Reagan always talks in a conversational tone, at USC Mondale was forced to shout over the din of boos and chants by hecklers, and straining his voice makes it even shriller and whinier. Sweating slightly in the Southern California sun with his shirt-sleeves rolled up, he looked and sounded like a candidate for sheriff.

The line about how he got the bags under his eyes was a good one, but he stepped on it. His response came out strident and challenging, rather than as self-deprecating humor, as he intended.

Reagan, on the other hand, never sounds strident.

"When he talks about his closeness to his wife, he's sharing his feelings about someone important to him, and people trust him because he's confided something to them," said Prof. Kathleen Hall Jamieson of the University of Maryland.

Jamieson, who has written a history of political advertising and communication in presidential elections, said, "He's comfortable talking about himself and he gives people some access to the private Reagan, which makes it hard to attack him. Mondale tries to talk about himself, but he's uncomfortable doing it because of his upbringing."

Reagan's voice itself helps ward off attack, Jamieson continued.

"He communicates compassion. Even when he says he's going to cut food stamps, he still sounds compassionate," she said. "He just sounds like a genuinely decent human being, always in control. But Mondale doesn't sound compassionate, and this works against his raising the issues of fairness and belligerence in foreign policy against Reagan."

One key to Reagan's political success is his skillful exploitation of the traumas of the past 20 years. After Vietnam, Watergate and the Iranian hostage crisis, after Lyndon B. Johnson, Richard M. Nixon and Jimmy Carter, the American people want to be able to trust their president, primarily because of the enormous power he wields.

And, as he demonstrated at the D-Day anniversary ceremony, Reagan also is unsurpassed in his ability to appeal to Americans' need for a moral reaffirmation of their pride in their country, which has been sorely buffeted in the past two decades.

Mondale, on the other hand, is a product of the school that believes that thoughts and ideas count, not how you say them.

"Reagan, like Franklin Roosevelt, thinks in terms of values and underlying passions, but Mondale thinks and talks in terms of programs, not values. So, he comes across as a professional politician, the programmatic product of the inside, liberal, Washington establishment," said a Democratic campaign manager. "People are looking for moral leadership, not political. Mondale talks about values the way the Brookings Institute does. You can hear the words, but not the music."

The tone of Mondale's voice is a big part of the problem.

"He can't do anything about his voice, he was born with it," said one campaign manager. "But he can work with forums in which he can speak in normal tones, like he was in your living room. Shouting just exaggerates the shrillness."

David Garth, the New York political consultant who did Mondale's ads in his victorious primary campaign last spring, thinks Mondale can look good on television, but has been spooked by his advisers.

"They've told him he's no good on TV, and that would make anyone nervous," Garth said. "He's a good-looking guy. He's not soft and flabby. Get him outdoors, cut out the frills so he's comfortable and have him talk right into the camera about jobs and arms control. His strength will come through."

If Mondale is a creature of programs and issues, Reagan is a creature of the camera, one who, in the words of one Democratic television specialist, "understands that politics is a performing art, not political science."

"Many politicians are bored stiff with the techniques of the media," said John Deardourff, a Republican campaign consultant. "But Reagan never tires of it. He studies it constantly, looks at tapes of his performances and works to improve them. He has a trained voice, an awareness of the camera and how it functions and doesn't get upset when it breaks down and delays things for half an hour.

Mondale, meanwhile, likes to say, "I'm not the hair-spray candidate," his metaphor for television.

"He hates the media and television people, and they have a hard time penetrating his campaign," a Democratic campaign consultant said. "He resists the idea that he's a candidate of the 1980s and seems to associate the techniques of television -- camera angles, makeup, memorizing statements at the beginning of a press conference -- with unmanliness. But if you don't learn it, you'll never be president -- or a good president."