UNANIMITY ERUPTED without warning in the frequently fractious boardroom of the AFL-CIO one day last month, signaling a change that may prove pivotal to the selection of the Democrats' 1984 presidential nominee.

To their own surprise, the leaders of 33 politically divergent unions who sit as the federation's executive council agreed to take an unprecedented step: to try to unite behind a single candidate before the caucuses and primaries begin. Their decision could substantially alter the shape of the presidential campaign.

It could be the making of a presidential candidate. "This could be the renaming of the Democratic Party," grumbles one presidential hopeful who knows he will never be the union leaders' first choice and senses defeat if organized labor indeed ever gets organized. "We may wind up having to call it the Democratic Labor Party."

It could, however, prove labor's political unmaking. Instead of showing its strength, the 15-million-member AFL-CIO could betray weakness by endorsing a candidate who goes on to lose in the primaries -- possibly because labor's own rank and file once again will not rally behind the choice of the union chiefs.

However it turns out, the plan will significantly change the way candidates campaign for the Democratic presidential nomination.

Although the party recently revised its rules to shorten the nomination campaign season, candidates now are likely to start campaigning earlier and harder than ever. Their first target will not be Iowans or New Hampshirites, but the 101 members of the AFL-CIO general board who would be empowered to deliver the federation's endorsement -- and the invaluable organizational support of computers, phone banks, voter lists and campaign workers that would come with it.

Failing that, the candidates may well seek, individually or in alliances of political necessity, to at least block the front-runner from receiving the required two-thirds vote.

And so Lane Kirkland brought his proposal up rather gingerly, seeking discussion but nothing so binding as a vote.

"We usually split in four or five directions, and we wind up fighting against each other," the AFL-CIO president said. "We would do better to act collectively."

"Noble objective," interjected Lloyd McBride of -- saw a Syr the Steelworkers. "But we need a harness -- something to prevent someone from laying hands on a candidate early. We need to stay hitched, so no one stampedes."

In the boardroom, the rest knew to whom McBride was pointing.

"Watch Wimpy -- he's our card-carrying maverick," one skeptical union president had cautioned as the meeting began. But in the discussion, "Wimpy" -- William W. Winpisinger of the Machinists, chief spearcarrier of the 1979 Draft Kennedy movement -- joined in exchanging vows of fraternity and fealty.

"The Machinists will stay hitched," he promised. Later, Winpisinger, who speaks mainly in exclamation points, elaborates: "I will make my pitch for my man (Kennedy) -- but if it doesn't fly, then, dammit, I will go by the decision!"

Even if it means leading his Machinists into Iowa and New Hampshire to work against Kennedy? "Yes! Even that! I may swallow hard, but I will do it! This SOB Reagan has made unity more important than any one candidate! I am pledging our union to the decision that is made!"

And one by one, as they go around the table, the union leaders find that they are united in their hope of forging a new political era of one-for-all-and-all-for-one. Solidarity, American style.

Glenn Watts of the Communications Workers, who backed Carter last time, says he is all for politicking the united way this time.

"At the moment I am not pushing anybody," he explains. "I think Mondale would make a fine president . . . I think Kennedy would make a fine president . . . We have got to consider who is the most electable."

Gerald McEntee of the American Federation of State, County and Municipal Employes says he is concerned that some union will go off on its own before the federation can act. He proposes a resolution asking that all unions refrain from endorsing anyone until a unified plan of action can be drawn. Kirkland and his assistants are surprised that it passes without dissent.

Joyce Miller of the Clothing and Textile Workers has a question: Once a candidate wins the support of two-thirds of the general board, what will happen to those who refuse to go along with the choice? "There is no way to hold those people in line," Kirkland concedes.

"It is a gamble, in a sense," he adds in an interview. "But if the federation does nothing, the unions will go their separate ways in the primaries, as usual. And then we will be fighting against ourselves, as usual. So really the question is not whether the trade unions will be involved in the primaries, but whether we will do it together."

In the corridors and meetings rooms of the AFL-CIO Northeast regional convention in the Hartford (Conn.) Sheraton, labor's lower echelons are also expressing a unity of sorts. They are united in thinking that something should be done -- but they are united as well in their uncertainties and doubts about whether their huge federation can really endorse as one and make it work.

"Lane Kirkland doesn't run union hall meetings," Robert Petronella of United Food and Commercial Workers Local 371 in Connecticut is saying."I have to run local meetings -- that's why I got these baggy eyes."

Petronella has just been asked if he thinks he could lead his local to work, say, against Edward Kennedy if someone else is endorsed. His response is a grimace.

"You see, I got a special place for Kennedy," he demurs. "So I don't know. I am a Democrat, but I am a labor official first. I am a labor official since I was a baby. So I got to go the way my state executive board goes. But I don't think my state executive board would ever do that, no matter what. I mean, after 12 years or something, now I've got to say (to my candidate) 'The hell with you, I found a prettier girl on the next block'? I don't see it.

"But don't get me wrong. We should look at it. At least Kirkland is trying. Under Meany they never did anything. But Kirkland is like Pope John. He has opened the windows. . . . We needed a little fresh air."

Others agree. "It would be difficult, very, very difficult," says Sy Cohen, director of the New York state AFL-CIO political action committee. "We don't want to get stuck with a lesser-of-two-evils situation again, like we had in 1980."

Meanwhile, back in the boardroom, the AFL-CIO chieftains stress to each other that they are not talking names. They are only talking procedures.

But they know, deep down, that the procedures they are talking about translate mainly into two names: Edward Kennedy and Walter Mondale.

They are the only two who seem at present to have a real shot at winning an early AFL- CIO endorsement, according to the heads of several international unions, local union officials, Democratic Party officials and even the other potential Democratic candidates.

These union and party officials note, in most instances, that the earlier the decision- making procedures, the greater the chances are that Kennedy can come away with the AFL-CIO's endorsement.

A candidate who has a big early lead in the polls stands a better chance of winning the endorsement, if the federation makes its decision on the basis of who is the most electable before the first caucus or primary is held.

Right now, Kennedy has just such a lead.

The last Gallup Poll gave him 45 percent to 12 for Mondale. Such early leads in the polls, however, often melt with astonishing rapidity once a campaign gets going and the public starts to focus on the foibles of the frontrunner (see George Romney '68, Edmund Muskie '72, and Edward Kennedy '80).

Still, under Kirkland's proposal, the early polls will be just about all that the union leaders have to guide them in determining public sentiment, he says.

Kirkland envisions that the AFL-CIO's general board -- which is composed of the presidents of each of the federation's 101 unions -- would convene in December 1983 or at the latest January 1984 to try to come up with an endorsement.

First, all candidates would be asked to appear before the board to speak and answer questions. "We'd just invite the candidates to do their best number," Kirkland says.

Then the general board would retreat to the privacy of its chambers to vote, leaving the politicos outside to await the puff of white smoke. Although a two-thirds vote would be required, the votes of the union presidents would be weighted to reflect the memberships of their unions. Thus the presidents of the 10 largest unions would control more than half of the votes of the board, and the 17 largest unions could provide the two-thirds margin.

If no candidate emerges with two-thirds of the vote, the federation's general board will likely decide to adjourn and reconvene to try again after a few early primaries have passed. United Automobile Workers president Douglas Fraser is among those who strongly favor delaying the endorsement until after the first round of primaries. ". . . December (1983) or January (1984) would be premature," Fraser says. "You've got to get a feel for what the people think."

Kirkland says it probably would be up to each union president to decide which candidate to back. He had toyed with the idea of holding a series of regional events for all the candidates -- but he has discarded that. He also flirted with the idea of having the federation's 1983 convention choose the candidate, but rejected that as too unwieldy.

An endorsement before the primaries begin would make it exceedingly difficult for someone to come from far back in the pack to win the Democratic nomination -- or from the outside, as Jimmy Carter did in 1976. This is no accident as far as Kirkland is concerned.

"It is questionable whether untested candidates are desirable or not, after our recent history," he says, referring to labor's disenchantment with Carter and George McGovern.

Kirkland and the other union presidents still chafe at the memory of about half of their rank and file voting for Ronald Reagan in 1980 because the leaders could not sell Jimmy Carter to their own union membership one more time.

"If we are not in it, if we wait until the convention is over, then we are stuck with other people's choices one more time," Kirkland says. "Why should we be stuck with other people's choices -- particularly if it coughs up candidates who are not saleable?"

If it turns out that labor lays itself open to charges of a return to bossism and machine politics, Kirkland says that is not all bad. "If there are any kingmakers around," he says, "we want to be in that number when the roll is called."