The Democrats have done it again.

For the fourth time in the last 15 years, they have revised their primary and caucus calendar and rewritten their delegate selection rules in bewildering ways.

Those who profess to know say the new rules help the cash-heavy, organization-rich front-runners like former vice president Walter F. Mondale and John Glenn. But the experts have been wrong before.

What is clear is that the rush of states to get to the front of the delegate selection line, and rules changes that encourage more winner-take-all primaries have combined to make it likely that the Democrats will know their champion by early April, well before the National Basketball Association and the National Hockey League know theirs.

The new system, the experts say, does nothing to diminish the attention candidates and reporters will lavish on the leadoff contests in Iowa and New Hampshire.

But it may reduce the aftershocks of those political events. It throws the spotlight more clearly on the Florida (March 13) and Illinois (March 20) primaries, and leaves room for the other big states--New York (April 3), Pennsylvania (April 24), Ohio (May 8), and even wallflowers New Jersey and California (June 5)--to play a role only if someone fails to wrap it up in a March blitz.

It also deals elected officials--members of Congress, governors, big-city mayors--back into the game, along with larger numbers of "old pro" party officials.

Theoretically, by staying uncommitted, these operators could turn this into the first brokered convention in 32 years. But at the rate they are signing up with candidates, chances are they will just increase the likelihood of an early verdict.

There will be slightly fewer primaries in 1984, 26 that are binding, compared with 31 in 1980. Some early caucus states like Michigan may be as important as any primary.

A few states still are jockeying for better positions on the nomination calendar, and the final timetable will not be set until some time this fall. But the general pattern is well enough known now that all the campaigns can make their assessments of strategy.

In early January, after Congress returns, the House Democratic caucus will pick 164 of its members as "uncommitted delegates," though many will have made their choices known. Senate Democrats will pick 28 delegates later in the spring.

The public game will begin once again in Iowa, with precinct caucuses on Feb. 20 or 27. New Hampshire will have the leadoff primary on Feb. 28 or March 6.

These dates are not yet firm because Maine is pressing to squeeze in its caucuses on March 4 and Vermont wants to hold a nonbinding presidential preference poll on March 6. If Maine and Vermont get their way, then Iowa and New Hampshire will probably go on the earlier of their pairs of dates, rather than the later dates sanctioned by the national party rules.

Iowa and New Hampshire have been the springboards for a succession of long-shot candidates, from George McGovern through Jimmy Carter.

They won the nomination by scoring unexpected successes in states such as Iowa and New Hampshire where one-on-one campaigning over many months can build personal followings for previous unknowns.

Reubin Askew, Ernest F. (Fritz) Hollings, Gary Hart and Alan Cranston are hoping the same lightning will strike them, and they will be targeting their limited money and manpower on one or both of those states.

Knowing the history, front-runners Mondale and Glenn will be matching the long shots dollar for dollar and organizer for organizer.

"Iowa and New Hampshire have become institutions," said Herb Hedden, director of delegate operations in the Glenn campaign. "Almost all the serious candidates will spend the maximum allowed by the Federal Election Commission in those states."

But Hedden, like many others, believes it will be harder for Iowa or New Hampshire to put a long-shot candidate into the nominating orbit in 1984 than it was before. The reason is that the results of those states will soon be buried in a mass of other delegate voting.

In 1980, the Iowa caucuses were on Jan. 21, and the New Hampshire primary was five weeks later--with only the Maine caucuses as a minor distraction in between. This time, Iowa will be at least a month later and only eight days before New Hampshire, a much shorter time to dominate the news.

The compression of the calendar was one of the big changes ordered by the latest national party rules commission, headed by North Carolina Gov. James B. Hunt Jr. Two of its major goals were to shorten the campaign season and to diminish the bandwagon effect of plurality victories by the "outsider" candidates in states like Iowa and New Hampshire.

As the frenetic summer schedules of the candidates show, the Hunt Commission failed in its aim of shortening the campaign. But rival strategists think it may have more success in putting the brakes on an Iowa-New Hampshire bandwagon candidacy.

Soon after the voters in those states have dropped the pebbles that send the first ripples across the political pond, the candidates will be thrust into the closest equivalent to a national primary the country has ever seen.

Between March 13 and March 20, there will be eight primaries and 14 caucuses, choosing almost 1,150 delegates, more than half the 1,966 needed for nomination. In 1980, only 13 states chose delegates in the comparable mid-March week, clear evidence of the rush to be at the front of the line next year.

Bob Beckel, Mondale's campaign manager, says "there is at least a 50-50 chance" that the identity of the nominee will be known by the end of the March 13-20 cross-country contest.

While the lineup of contests in that critical stretch has something of a southern tilt--with eight Dixie and border states in the mix--Beckel said he considers it "broadly representative of the country." The battlegrounds range from Hawaii to Puerto Rico, with the critical contests probably coming in two states that are as diverse in their makeup as any in the land: Florida and Illinois.

Florida has a home-state favorite son in former governor Askew, but Glenn and Mondale are mobilizing full-scale efforts in what Hedden calls "a prime breakout possibility for somebody." Mondale is rated the early favorite in Illinois, but will certainly face a formidable challenge there.

Those states are viewed as particularly critical, partly because of their size, partly because of the kind of primaries they will hold. Both will offer voters a chance to vote directly for delegates, a system that tends to give a big advantage to any candidate who can establish even a narrow plurality lead over his challengers.

In Illinois in 1980, for example, Carter won 65 percent of the "beauty contest" popularity vote over Sen. Edward M. Kennedy (D-Mass.), but took 165 of the 179 delegates.

Illinois was one of only two states that year, along with West Virginia, that the national party granted an exemption from the rule requiring proportional representation of the popular vote in both primary and caucus states.

The Hunt Commission, in another of its major changes, allowed other primary states to use the winner-take-all or direct-election system. Florida, Pennsylvania, Maryland, Ohio, California, and New Jersey all have taken the opportunity to upgrade the importance of their contests by this means.

The Hunt Commission also gave states a winner-take-more option, allowing them to give one delegate to the winner of each congressional district primary or caucus before apportioning the remainder of the delegates according to the popular vote. Georgia, Puerto Rico, North Carolina and New York have adopted that for their primaries, and Hawaii, Arkansas, Kentucky, Colorado and Idaho will use it for their caucuses.

The notion behind the winner-take-all and winner-take-more rules is to speed a consensus around a leading candidate, rather than prolong the struggle by keeping stragglers alive.

That rule change dovetails with a longstanding Federal Election Commission provision cutting off federal matching funds from any candidate who receives less than 10 percent of the vote in two successive primaries. Together, the two factors virtually dictate that the long-shot candidates bet almost all their chips in the very early contests--or they may not survive to compete in the March 13-20 pseudo-national primary.

"There is very little turnaround or recovery time for a candidate who stumbles," Beckel observed.

John Rendon, a political consultant who monitors the rules changes for the AFL-CIO, says that many of the long shots will be forced to "a segmented strategy," picking a few early states as tests of their strength and hoping that the press and television grade them on their showing there, rather than in the overall delegate-counts.

But even if somebody scores a Carter-type upset early, it may be tough to go all the way, for the financial and organizational requirements of that mid-March period are awesome. Beckel, whose candidate is currently ahead in both money and organization, emphasizes but probably does not exaggerate the consequences of that "compression."

"You have to have a large crew of good organizers because you won't have time to shift a few people from place to place, as campaigns have done in the past," he said. "That will enhance the value of endorsements from groups like NEA the National Education Association and the AFL-CIO, which have ongoing organizations of their own in so many states.

"It also enhances the value of endorsements from politicians with their own political organizations, especially in caucus states," which dominate the first half of the delegate selection calendar. "All this, we think, works to our advantage," Beckel said.

It is no accident that the rules have taken this form.

When the Hunt Commission was meeting, in the aftermath of Carter's defeat, there was a "never again" feeling about such outsider candidates. Mondale and Kennedy supporters and AFL-CIO operatives were influential in the commission decisions, as were elected officials anxious to reclaim their place of influence in the convention hall.

When Kennedy dropped out, Mondale became the main beneficiary of those rules changes. But some observers think that by the time the primaries start, Glenn will have enough money and organization in place to share in the decided advantage over the long-shot candidates.

Glenn hopes to make up for some of Mondale's organizational edge by using television, and others may be forced into the same tactic--if they can afford it--by the multiplicity of simultaneous contests.

"So much is happening in a short time," Hedden said, "we'll use a large percentage of our resources on media."

Rendon agrees that "the primary states will probably see television campaigns, and the candidates will concentrate their organizational resources on the caucus states, where that can be decisive."

Another of the big rules changes was the decision to set aside 566 delegate seats--one-seventh of the total--for elected and party officials, chosen outside the normal primary and caucus processes. These "super-delegates" are allowed to be uncommitted, and theoretically could be a power bloc large enough to swing the convention one way or another.

But Hedden observes that "there will be fewer uncommitted delegates than we guessed last year. A large percentage" of the likely delegates among members of Congress, governors, big-city mayors and party officials are going to Mondale, he said, "and most of the rest Glenn is getting."

Thus, in his view, and that of others, the "super-delegates" are likely to be a force for squeezing out the long shots and forcing an early decision between the front-runners.

Nevertheless, no one rules out the possibility of a protracted struggle. If no candidate emerges with a clear lead on March 20, the remaining major events are spaced widely enough to make each one a battleground of its own, with time for some fund-raising in between.

New York could be the next big showdown. Party officials, led by Gov. Mario M. Cuomo, want to push the primary back to May 22, but they have run into resistance in the Republican legislature and the current betting is that New York will stay on April 3. The other major states, in their calendar order, would be Pennsylvania, Texas, Ohio, New Jersey and California.

California Democrats, unhappy that the nomination almost always seems to be settled before they get to vote, toyed with several ideas for picking some or all of their horde of delegates early, but all the schemes seem to have died.

If there is still no verdict on June 5, when the last votes have been counted in California, the new rules set the stage for more uninhibited horse-trading on the San Francisco convention floor than anyone has seen since Adlai Stevenson won a third-ballot victory in 1952.

In addition to the 566 "super-delegates," all of whom will be officially unpledged regardless of any verbal commitments they may make, it will be easier for pledged delegates to shift to other candidates than in the past. The Hunt Commission scrapped the 1980 rule allowing presidential candidates to jerk the credentials of any delegate who threatened to bolt, and said, in effect, that the delegate will be the arbiter of his or her own conscience when it comes to voting.

However, the rules still give candidates the prior right of approval of pledged delegates, and most will use that right to assure that loyalists are in their ranks.