"I think starting with Labor Day, the real fight begins," former vice president Walter F. Mondale said last week. "In a sense, we've been wading in the lily pads up to now."

As the crowded field of contenders for the Democratic presidential nomination moves into the last months of jostling for position before voting begins in the early 1984 primaries and caucuses, Mondale and Sen. John Glenn (Ohio) appear increasingly to be battling only each other for the lead.

The two front-runners so far have dominated the presidential field of six. Yet the other four--Sens. Alan Cranston (Calif.), Gary Hart (Colo.) and Ernest F. Hollings (S.C.) and former Florida governor Reuben S. Askew--also are coming out of August's days of political respite and reassessment with newly calculated strategies for making the race theirs to win.

Two other Democrats, the Rev. Jesse L. Jackson, the civil rights leader, and former senator George S. McGovern, the party's 1972 standard-bearer, are threatening to join the chase. And if the two front-runners were to collapse, pressure would increase on Sen. Edward M. Kennedy (Mass.) to enter the race.

Mondale and Glenn provide the party with a distinct choice. One bases his appeal on traditional Democratic liberalism. The other offers a more centrist appeal that reaches out to independents and even to Republicans.

Mondale, the traditional Democrat, has the best organized and best financed campaign of the six--and he is expected to widen that advantage significantly in coming weeks by winning the endorsements of two huge labor groups. Both the AFL-CIO and the National Education Association, with 15.4 million members between them, are expected to pledge their support to him in an effort to demonstrate their own political clout.

Glenn, the centrist, has pulled close to Mondale in opinion polls, fueled mostly by voters' memories of his heroics as America's first orbiting astronaut. That aura may well be boosted dramatically next month by the release of "The Right Stuff," the Hollywood version of author Tom Wolfe's account of America's first seven astronauts.

The other four declared Democratic candidates enter this crucial stage of the race determined to continue their vigorous campaigns after making a few alterations in their original battle plans. They take comfort that the two early front-runners, despite large leads, are not without significant political flaws.

Cranston made some early headway by wrapping his candidacy around the nuclear freeze movement and showing strength in straw polls. But he paid a stiff price for his efforts in those officially meaningless but sometimes influential straw ballots, and his campaign is heavily in debt.

Hart said he plans to unveil a new theme and alter old perceptions in an effort to regroup after some tactical blunders and organizational setbacks got his "new generation" candidacy off to an uneven start.

Hollings said he hopes to make a dent in Maine's Oct. 1 straw poll and then use that showing to convert the assets of a quick tongue and a sharp mind into popular appeal.

Askew traveled to all 50 states, even before 1983, and had early success in raising money, chiefly in his home state. He appears to be concentrating on the New Hampshire primary.

Jackson is the wild card in the contest. Political analysts say they believe the director of Operation PUSH People United to Save Humanity , who may enter the race this month, is likely to attract black support that otherwise would have gone to Mondale, providing Glenn with an even greater chance to score well, especially in the South.

The next few months will see an increase in campaign activity by all candidates and a greater push to raise money before the caucuses and primaries begin. Fund-raising is especially important in coming months, because the heavy schedule of caucuses and primaries will allow less time to raise money during the election year.

Iowa holds its caucus on Feb. 27 and New Hampshire follows with its primary on March 6. Then comes the week of March 11, when about 20 states will hold caucuses or primaries, including 10 on March 13--which is being called "Super Tuesday"--and seven on March 17. Candidates without adequate financing before this rush will be hard-pressed to keep going.

Part of the campaign story can be told in numbers. The national polling percentages have shown Mondale mainly in the low 30s to low 40s, Glenn in the high 20s, and the rest well behind.

Mondale continues to lead his opponents in fund-raising. In the first six months of 1983, he raised $5.1 million, while Glenn raised $2.1 million. Cranston took in $1.1 million, Askew $990,643, Hart $830,834, and Hollings $452,579. The Mondale Campaign

Mondale strategists say they have accomplished all they set out to do this year: win the support of key party officials and institutions, organize key states and raise more money than anyone else.

But while his superior organization and finances are valuable assets, Mondale has yet to demonstrate after more than a year of campaigning that he can generate the sort of enthusiasm that can carry a candidacy through hard times. This has led opposition strategists to comfort themselves in the theory that if Mondale stumbles early, his candidacy will crumble.

Not so, countered James A. Johnson, Mondale's acting campaign manager.

"I think we are in good shape for the long haul for two reasons," he said. "First is that Walter Mondale has shown himself to be the best prepared and most effective candidate. And second, we've built the most effective organization and political base."

Still, the Mondale campaign remains haunted by June's Wisconsin straw vote in which Cranston defeated Mondale. "Our straw vote prospects lost out to the sunshine," a Mondale strategist said in a moment of summertime candor that revealed as much about the depth of pro-Mondale enthusiasm as it did about Mondale's organization.

Mondale strategists vowed that they will never again be out-organized--and that, they have said, is where labor's endorsements will help mightily.

His opponents, who sought first to win labor's backing and then to block Mondale from getting it, now argue that the endorsements will be a distinct detriment to Mondale, especially in the South. The endorsements, they say, will emphasize what is already a Mondale vulnerability: that he is regarded by some as beholden to special interests. But early county caucuses in Florida have shown that labor also could deliver impressively for Mondale in the South--at least among party stalwarts.

Glenn political director Joseph Grandmaison observed: "Labor could mean $20 to $25 million for Mondale . . . . If Mondale has an early setback, organized labor can come in and resuscitate his candidacy. And that gives Mondale an advantage no one else has." The Glenn Campaign

Glenn's campaign has followed its own rhythm, one that generally has avoided direct confrontations with his opponents in straw polls and that deliberately has kept him off national television. Labor Day finds him in an enviable position, but he and his staff recognize that he now faces several important tests.

The first is to define himself. Glenn's polls show that while he is well known and well liked personally, he is almost a blank canvas politically. "We see the post-Labor Day period as a time of direct communication in which John Glenn begins to fill in the blanks for the average voter," said one Glenn adviser.

Glenn said he will speak out increasingly on major issues as he tries to project himself as a man for the future. But opponents say they wonder whether Glenn's middle-of-the-road policies will prove attractive to the traditional Democrats who vote in primaries and caucuses.

The second test for Glenn is to prove that he can take the heat of a presidential campaign. He has not been exposed to the kind of scrutiny his staff expects in the months ahead, and there have been warning signs lately that this could be a difficult period.

He was criticized for a poor performance before the House Democratic Caucus in July, and last week he retracted a mid-August statement that he, as president, would not retaliate if the Soviet Union sent a nuclear "demonstration shot" into a sparsely populated area of the West--this from a candidate aiming to do well in the West and South.

Finally, other Democrats say that Glenn must prove that his organization is ready for the hard primary and caucus season. His advisers assume that Mondale will have the support of organized labor, which puts more pressure on Glenn's organization. But doubts persist in many states about the quality of those units.

Grandmaison dismissed those criticisms. "All I can say is that I'm absolutely confident that we will be competitive at the time we need to have an organization in place," he said.

If Glenn passes his tests, Mondale's advisers know they are in for a drawn-out contest. "Then it can be a tight two-man race all the way," Johnson said. The Cranston Campaign

Cranston was the surprise of the campaign during the first half of 1983, thanks to a good issue, a shrewd strategy and sheer hard work. But the summer has been less kind. In July, he reported to the Federal Election Commission that he was about $300,000 in debt, and in August, he gave up hope of winning the AFL-CIO endorsement.

Now he has thrown everything into the Oct. 1 Maine straw poll. "We want to reestablish that it is indeed a three-way race," said Sergio Bendixen, Cranston's campaign manager. Cranston's other immediate goal, he said, is to raise $1 million in a targeted autumn television appeal.

Cranston's victory in the Wisconsin straw poll in June and his second-place showing in Massachusetts appeared to spring the California Democrat from the pack of 1984 dark horses. But his inability to translate those performances into money or stronger popular support has left other Democrats wondering about his future.

Bendixen had said publicly that Cranston's goal is to eliminate Mondale by the end of March and then dispose of Glenn in the late primaries. But he has since admitted that this is unrealistic.

Instead, Cranston is counting on the support of liberal party activists, who play a disproportionate role in the nomination process, to keep his candidacy moving. The Hart Campaign

Hart is expected to continue his quest for the presidency this fall, bolstered by a new campaign theme and a new determination to define his candidacy. His top advisers say that he will present himself as a new generation of Democratic centrist, not just a younger liberal alternative to Mondale and Cranston, as he has been labeled for much of the past year.

This year has not been all that the Hart strategists had hoped, but they point to grass-roots organizational successes in Iowa and New Hampshire and on more than 100 college campuses.

Hart's setbacks came in part when he competed in the straw polls in Massachusetts and Wisconsin and wound up buried deep in the pack, behind Cranston. That made fund-raising harder. His campaign is in debt, but his backers vow that it will be in the green by the end of the year.

Hart appeared to function more as his own campaign manager in the early months than did any other candidate. While he successfully managed McGovern to the 1972 presidential nomination, his divided attention in 1983 appeared to result in uneven performances both on the stump and in the organization.

The good news for the Hart faithful is that his strategists, led by new campaign manager Oliver (Pudge) Henkel, have recognized past shortcomings and said they have moved to correct them.

"We haven't defined our candidate and honed our candidate's message as well as we will," said Henkel. ". . . Gary is far closer to the center than he is perceived."

"We let the contests in the straw polls position Gary for the public. In Wisconsin, for example, we let Alan Cranston define our candidacy by saying he was out to defeat Hart to become the liberal alternative . . . and that's why we were seen as losers."

Hart's goal now is to show that he can offer new solutions to traditional problems--for example, that his ideas on military reform can achieve more formidable military strength than can Glenn's military spending proposals.

His overriding goal is staying alive through the critical March 11 week of the primary season. "Anybody who's alive after the week of Super Tuesday is going to have a great shot at the nomination," Henkel said. The Hollings Campaign

Hollings has been trying to shake loose from the charge that he is running for vice president, and has chosen the Maine straw poll as the occasion to make that clear.

Until recently, he simply was moving around the country, arguing that the Democratic Party's ties to special interests have helped bankrupt the country and that the federal budget deficits are preventing the U.S. economy from making the kinds of investments needed to restore America's competitiveness in the world.

But last month, after a strong appearance at the National Governors Association meeting in Portland, several leading state officials urged Hollings to enter the Maine straw poll contest. He has lined up a cross-section of Maine Democrats and plans to spend 10 to 12 days in the state.

"I think Hollings comes out as second choice for a lot of people," said Hollings campaign manager Billy Keyserling. "The question is, what is the opportunity we have or create for ourselves to break out. Placing in the middle in Maine could be the beginning of that." The Askew Campaign

Askew traveled farther, earlier, than any other candidate for the 1984 nomination. He also began mining campaign dollars early in Florida and continued to work his home state for funds this year. As a result, he now is much better off in the bank than he is in the polls.

Askew, who was known for his progressive stands in Florida, including his support of busing to aid desegregation, has staked out a moderate-to-conservative stance in the national presidential field. He opposes abortion by choice and the AFL-CIO's domestic content legislation.

Askew campaign director James Krog said that organizationally, Askew has gotten off to his best start in New Hampshire. "We have to show early that we can get some delegates outside the South," Krog said. " And then we have to do a little better in New Hampshire.

". . . Right now there are two tiers: Mondale and Glenn are in the first tier; everyone else is in the second . . . . But it's like getting into the Super Bowl--first you win your division and then you get to go up against the winner in the other division. Between Mondale and Glenn, there will be only one winner. All we have to do to get in the playoff is win our division."