In the workaday world of presidential politics, where images are shaped with videos, cameos and winks and nods, the Mondale-Ferraro Committee's new chief of message-making surprised a July brainstorming session by interjecting an unusual campaign concept.

"Let's talk about truth," Richard C. Leone said to Walter F. Mondale.

"It comes down to leveling with the American people," Leone went on, according to others who attended the session that had been convened in the Mondale living room at North Oaks, Minn., to plan his Democratic National Convention acceptance speech. "What truth do you want to convey? Where has Reagan not been leveling with the American people?"

"Deficits," Mondale replied, quickly picking up on the concept and making it his own. "This incredible deficit and the fact that there will have to be more taxes to pay for it no matter who is the next president."

So it was that Leone began his job as Mondale's chief message-maker, as the Democratic presidential nominee wound up using his convention speech to jar conventional wisdom. He altered the landscape of the campaign battleground with his promise that as president he would have to raise taxes -- and that so would Ronald Reagan.

Leone, little known outside the Mondale hierarchy, is at the center of Mondale's long-shot run for the presidency. Mondale's polls have showed all year that people think he promises too much; Leone argued that taking a tough position on a tough issue was the only lasting way to counter that.

Leone has been granted a sweeping Mondale campaign franchise that is unusual to presidential politics. He has no title other than "senior adviser" and is listed on the campaign chart under the category of "issues." Leone is in fact in charge of "the message" -- what the candidate says and how it comes across to the public.

His task is to coordinate all things that have to do with how the public comes to know Mondale: issue development, speech writing, television advertising and public-opinion polling.

"I believe to my very core that we are going to have a powerful message to tell and we will tell it in a powerful and dramatic way," Leone said just before the fall campaign began. ". . . The message that we have to bring to people on the deficit is much more complicated than Reagan's. And we have to be very smart to get it through so people can see it for what it is."

Much to their political chagrin, Mondale kicked off the fall campaign on Labor Day with the sort of message fiasco that Leone's wide-ranging role was created specifically to prevent.

In a display that dismayed Democratic pols from coast to coast, Mondale and running mate Geraldine A. Ferraro opened their fall campaign to the sound of silence, walking Manhattan's Fifth Avenue canyon in a "parade" that attracted no crowd -- and just a handful of participants. The day ended in California at a rally where Mondale was plagued by problems with the microphone.

Distraught Mondale aides say that Leone just didn't pay much attention to the New York event; he had spent most of his time coordinating the theme and message of Mondale's remarks at a midday rally in tiny Merrill, Wis. Those remarks carried a powerful rich-versus-poor theme, but on the television news shows it was drowned out by the overriding message that Mondale's Labor Day opening was a fiasco.

"Dick learned a painful lesson," said one of his associates. "We all did."

It has often seemed that the Mondale staff has learned the same lesson time and again during the black-and-blue primary season. The organization directed by Mondale campaign chairman James A. Johnson often seemed to pay little attention to message -- the collection of images that the public was getting from Mondale and his operatives. When they did think about the messages they were sending, they were often woefully wrong, Mondale aides said privately.

In the last days of the primary season, Mondale had nothing but praise for his campaign in New Jersey -- where Leone was heavily involved. But he was heard by top aides to grumble on several occasions about a California campaign program that left him heavily scheduled but without a clear sense of what he was trying to say. "Why am I doing this event?" he asked. "What is the message we are trying to send?"

Several days after that June 5 primary wrap-up, Mondale discussed the problem with Johnson as they sat on the back porch of a Long Island house Mondale was using for a summer respite.

"You are dealing with too many people and so am I," Mondale said, according to Johnson's recollection. Johnson agreed. Mondale said the campaign needed one person who would be responsible for coordinating all facets of the campaign message.

A couple of days later, Johnson suggested Leone for the job. He had known Leone for two decades, since his post-graduate days at Princeton's Woodrow Wilson School of Public and International Affairs. In 1968, they worked opposite each other, as Leone aided Robert F. Kennedy's campaign while Johnson worked for Eugene McCarthy. In 1970, they taught a course at Princeton, "Introduction to Public Policy."

Leone is a New Jersey politician who looks about as much like a New Jersey politician as, say, Woodrow Wilson, and has savored no such personal electoral success. After being appointed treasurer of New Jersey, Leone ran third in the 1978 Democratic senatorial primary that preceded the election of Sen. Bill Bradley. Away from politics, Leone served as president of the New York Mercantile Exchange and of Atlantic Commodities Inc., a trading company with ties to the Amerada Hess Corp. oil company.

Leone's political success has been mainly behind the scenes. Twice he guided Democrat Brendan Byrne to come-from-behind gubernatorial victories in New Jersey.

In the state's presidential primary, Johnson and other top Mondale advisers credit Leone with pressing Mondale toward his successful strategy of going beyond his safe base of traditional Democrats and pursuing the "Yuppies" (young urban professionals) who were supposed to be Sen. Gary Hart's support base, with coordinating Mondale's overall message of speeches and ads, and with singling out Hart's opposition to gun control laws for special attack.

Leone's role puts him in the cross fire of competing and individually prominent Democratic advisers. In the issues-development area there is Bill Galston, former Carter White House deputy national security affairs adviser David Aaron, Susan Estrich, who was executive director of the Democratic Platform Committee and a former aide to Sen. Edward M. Kennedy (D-Mass.), and Carter administration National Security Council staff member Madeline K. Albright.

Mondale's media specialists include Roy Spence, the Texan who once had the Mondale account all to himself; David Sawyer, who designed the anti-Mondale ads for Sen. John Glenn's presidential campaign; Judy Press-Brenner, a consultant who is being given a central role in Leone's office; ad man David McCall, whose Madison Avenue credits include the Mercedes-Benz automobile account, and Frank Greer, a Washington media campaign consultant.

Mondale's polling experts work under the overall direction of pollster Peter Hart and include William Hamilton, who conducted Glenn's campaign surveys; Dotty Lynch, who polled for Gary Hart; Edward Reilly of Boston, and the Penn & Schoen organization, which Leone knows through his friendship with their New York benefactor, media expert David Garth.

With such a stable of political thoroughbreds, each used to running on his and her own turf, Leone runs the risk of making high-strung enemies in high party places. But so far, in the early stages of the race, those involved say, it hasn't happened.

"We needed someone who could pull it all together," said one of the well-known political experts in Mondale's message consortia.

"He is a message-oriented person who has the confidence of the candidate and who understands how it all fits together. We don't have time to fight among ourselves. Because we've got to move voters quickly -- or we'll lose."