The federally financed transition office close three weeks ago, and the Secret Service men departed. Six months after he left the public payroll for the first time in 20 years, former vice president Walter F. Mondale is wholly a private citizen, one of those six-figure-salary Washington lawyers who attends stimulating conferences on three continents.

He looks terrific, tanned from a fishing expedition with his two sons. He is rested, relaxed, even a bit reflective. And he hopes the condition won't last.

By next summer, Mondale says, "I'll be on the trail almost full-time," campaigning for Democratic candidates in the mid-term election. His travels will be financed by a personal political-action committee that raised almost a quarter-million dollars before it sent its first direct-mail appeal last month.

And after that, there will be the 1984 Democratic presidential derby. Mondale figures to be one of the early front-runners in a vast field that may include senators Edward M. Kennedy, John Glenn, Gary Hart and Joe Biden, governors Jerry Brown, John Y. Brown, Jay Rockefeller, and Hugh Carey, former Carter cabinet colleagues Moon Landrieu, Reubin Askew and Robert Strauss -- all of whom see visions of Rose Garden through the aura of Ronald Reagan's current halo of popularity.

Mondale, for one, is convinced that the nomination will be worth the scrap. He says the massive tax-cut bill Reagan pushed through Congress in the past seven days will come to be seen not as his greatest triumph but as "his worst mistake." It will, he says, put intolerable pressure on the Federal Reserve Board as the sole agent in the fight against inflation. The resulting persistent high interest rates will not only choke the American economy but "do more harm" to U.S. relations with the European allies and Japan than Russia could contrive on its own to accomplish.

Mondale really believes that. But even if his forecast is faulty, he will go for the brass ring of the nomination in 1984. For him, that year is up or out. He passed up a chance to compete for his old Senate seat from Minnesota in 1982, even though he still maintains a voting residence in the state. His standing among party activists and the leaders of allied interest groups is never likely to be higher than it is now. At 53, he believes 1984 has to be his year.

But aside from his gamble that Reaganomics will fail, it is far from clear just what Mondale sees himself -- or his party -- offering the voters. In an interview last week, just before he flew of to Aspen for a seminar on U.S.-Soviet Relations, Mondale repeated the statement that he had made in a post-election interview: "We [Democrats] were sounding awful stale."

He says he has read and traveled widely these last six months, in an effort to refresh his own thinking: eight days in Europe, including a long look at NATO and its defense theories; an energy seminar with oil men and investors in California; many conferences on the domestic economy. In the fall, he is going to China, Japan and Korea.

Deliberately, he says, he has spent much more time in the South and the West, with conservative economists and businessmen -- looking at key issues from a perspective other than the one he learned as a disciple of Hubert Humphrey and Jimmy Carter.

But the differences are not yet apparent in Mondale's public utterances. His speeches have been given to "safe" audiences, the National Education Association and the Urban League, at the L.B.J. School in Austin and Brandeis University.

In them, he tips his hat to the current fashions. "Porgressives have learned some lessons," he says, but the lessons are the obvious ones: inflation is important and regulation burden-some. "Where government has been clumsy, or expensive or intrusive -- we should make government better."

But, mainly, he defends the causes he has always defended: nutrition and child-care programs, education, legal services, civil rights, voting rights, aid to Israel. His answer to the dilemmas of Social Security is the same as it was in the last campaign: "That Social Security check -- and the way it is figured -- should be as sure as the sun coming up in the morning."

Mondale says that the product of his intellectual re-examination will be more systematically displayed in three speeches he plans to give at prestigious public forums this fall, spelling out his approaches to the economy, foreign policy and the issues of social justice. "Fundamentally, what the Democratic Party needs is a fresh, unifying set of principles that the public can trust and that will provide our agenda when we come back into office," he says.

It remains to be seen how close Mondale will come to meeting that standard.

In the meantime, there is a possible clue to his hesitancy in breaking with which he wound up his most important and best-received speech of the last six months, the one he delivered to the Urban League convention in July.

"We can now porve," he said, "that we weren't as bad as they [the voters] thought we were, and that the other crowd is worse than we thought they would be."