Democratic presidential nominee Walter F. Mondale kicked off his fall campaign from coast to coast today, declaring that the November election is a "high-stakes" decision that will test the mettle of the American people.

With vice-presidential running mate Geraldine A. Ferraro at his side, Mondale told an enthusiastic crowd here, "We're not a reckless people, we're a strong people . . . a principled people . . . . We are not a fragile people . . . . We're not a people who steal from our children. We're a people who invest in our future."

Calling for an end to the "uncaring, icy indifference" of the Reagan administration, Mondale added, "We're not an unfair people, we're an honest, aboveboard people . . . . We're not a selfish people, we're a caring people."

Mondale, starting the fall campaign farther behind than any challenger since former senator George McGovern (D-S.D.) in 1972, planned to come out today as "Fighting Fritz" and take the battle directly to President Reagan.

He did that this evening at an airport rally in Long Beach, Calif., where he animatedly and forcefully railed against Reagan on the nuclear-arms and fairness issues and won strong applause from about 2,500 supporters.

Mondale criticized Reagan for not meeting with any Soviet leaders during his term and for being the first president since the end of World War II who has not negotiated an arms-control agreement. "Enough is enough," Mondale said. "Let's get a president with some sense."

He also lambasted Reagan's claims of economic recovery. "We all know we're living on borrowed money and on a borrowed and phony prosperity," Mondale said.

On the fairness issue, he said, "There is a streak running through this administration's policies that I believe offends the shared American notion of justice," and he criticized Reagan as hostile to the middle class, the poor and downtrodden.

On hand at the airport were Sen. Gary Hart (D-Colo.), Mondale's major challenger for the Democratic nomination, and Sen. Alan Cranston (D-Calif.) and Los Angeles Mayor Tom Bradley.

Hart told the crowd in impassioned introductory remarks for Ferraro that Reagan "must be stopped . . . . This is a watershed election . . . . We cannot afford to lose. I believe an upset is in the making."

Ferraro did some fighting of her own in Wisconsin as she criticized Republicans for "self-conscious patriotism that's made on Madison Avenue" and self-righteousness in religion, saying she rejected "any new tyranny over the minds of both men and women" that would result from imposing one group's religious views on others.

"Mr. Reagan talks about freedom and he should," she said. "But there's no freedom more important than the liberty to think as you want, pray as you believe, and the liberty to be left alone. Mr. Reagan is for limited government -- except when he wants government to affect our most private decisions and beliefs."

Wearing a blue-and-white farm cap and a plastic raincoat, Ferraro condemned Reagan's "jokes about bombing the Soviet Union" and his alleged coolness toward civil rights laws, while contrasting the careers of Mondale and the president.

"While Ronald Reagan was making movies, Fritz Mondale was making history as a champion of civil rights," she said. "While Ronald Reagan was host of 'Death Valley Days,' Fritz Mondale was trying to get Medicare passed for senior citizens. While Ronald Reagan was serving as the spokesman for the General Electric Corporation, Fritz Mondale was a spokesman for the average American family."

Mondale also hit Republicans on the religious issue, saying: "Those who seek to inject government and politics into religion lack confidence in the wisdom and the decency and good sense of the American people. They seem to be saying that the people can't be trusted . . . . I want a government operating on your side, opening doors of opportunity, not looking over your shoulder imposing dictates of conformity."

Mondale and Ferraro were greeted in Merrill by several thousand persons, who endured rain to line the parade route and roads along which the campaign caravan traversed the hilly farm lands.

Hours earlier in New York, it was an entirely different story. There, sparse crowds provided only a puff of enthusiasm for what Mondale aides had predicted would be a "smoking" campaign kickoff at the traditional Labor Day parade up Fifth Avenue in Manhattan.

Mondale and Ferraro marched at the head of the procession, flanked by AFL-CIO President Lane Kirkland, New York Gov. Mario M. Cuomo and Sen. Daniel Patrick Moynihan (D-N.Y.).

A brass band blared John Phillip Sousa marches, as members of the carpenters union hoisted blue-and-white "Mondale-Ferraro" signs toward the tops of the skyscrapers hovering over the route.

"Where's the crowd?" reporters yelled to Cuomo when the line of march paused in front of the New York City Public Library.

"They're all in church," Cuomo responded with a smile.

New York Mayor Edward I. Koch did not appear at the parade. Mondale aides said they believed that he was at the beach.

But former president Jimmy Carter, Mondale's old boss, was in New York, getting front-page newspaper headlines and sharing top billing with Mondale on the local early-morning television newscasts.

Carter was beginning a week-long stint helping to rehabilitate an apartment building on the Lower East Side. The effort was sponsored by a nonprofit Christian organization.

Rather than fleeing the ghost of Carter, Mondale was chasing the shadow of Reagan. The opening-day sweep to California brought the Democrats to the state where Reagan opened his campaign today. Later, Mondale planned to fly to Salt Lake City to address the American Legion convention Wednesday. Reagan is to speak there Tuesday.

Mondale strategists describe this game of presidential pursuit as a way to simulate head-to-head debates, sharpen direct comparisons between the candidates and underscore Democrats' willingness to take on Reagan in Republican strongholds.

But today's start-up also highlighted the fact that Mondale is seeking to pump up his campaign virtually everywhere in the opening days of September, in part because Reagan leads him in most states. Whatever geographical strategy Mondale has for winning this election will not become clear until later, aides said.

The opening in Manhattan was a bow to organized labor's traditional holiday and a chance to show off Ferraro in her home town. But to get to Wisconsin in time for the parade there, Mondale and Ferraro had to appear in New York early this morning. Aides blamed the early start for the small turnout.

In Merrill, a north-central Wisconsin town of about 9,500 people, where corn and dairy farming and pupl making pulp and paper products are the main ways of life, Ferraro made two minor slips.

At one point, she accented the last instead of the middle syllable in the last name of one of the heroes of the state's progressive political movement, Robert LaFollette. And when she asked voters' support on her own mission, she said, "You will also be making history when I stand in the Capitol and take the oath as the first woman president of this great nation."