The event he would have held if he hadn't been suddenly called upon to announce his surprise decision not to run in 1984 explains why the constituency of Edward Kennedy regards him as irreplaceable and won't be rushing to embrace any of the others who are still at the starting line.

Until the word leaked out that it was no go, Kennedy was scheduled to hold a news conference with the families of the four American churchwomen who were murdered in El Salvador two years ago today.

It was the kind of risky, thankless cause that Kennedy took on, and the kind of thing that none of the others would do.

If he has a "character" problem, he also has a unique reputation as the champion of the widow and the orphan, the grieving and the dispossessed.

But, as with everything he does, his announcement stirred conflict. For every Democrat who feels relief that a replay of the 1980 disaster is out, there is one who mourns that the stalwart liberal is to be sidelined.

And for everyone who believes that he was talked out of his dynastic, admitted -- and not permanently renounced -- ambition to be president, by his three children, there is somebody who cynically points out that exit polls in his native Massachusetts showed that his constituents, like his children, would rather have him in the Senate than in the White House.

The three children, blond Kara, smiling Teddy Jr. and the elfin Patrick, were in the front row at the announcement ceremony, which was accomplished in the Kennedy way, with high style, high feeling and a huge gallery. Even sister Eunice, the most royal of the family, was reported to be relieved.

His life will be easier. Democrats can't say that the big fellow didn't get out of the way soon enough--the others have two years to woo his people.

As he shouldered his way through the mob in the Labor Committee hearing room, thunderous applause broke out. He has lost as much weight as Richard Nixon deemed necessary for the nomination. He was a bit misty-eyed but at ease, certainly happier than the flushed and flustered man who announced his candidacy three years ago in Faneuil Hall.

Word of his withdrawal began trickling out late Tuesday.

The timing was a surprise. Preliminary planning was well advanced. The polls he had commissioned in primary statesshowed, according to his staff, that he was well ahead and suffering no increase in "negatives."

In his Senate campaign, he ran a series of curious, maudlin commercials to the effect that while he was not, in the words of one old man featured in them, "a plaster saint," he was capable of enormous personal kindness. They were widely seen as the first attempt to erase the Chappaquiddick stain, which they did not address.

The politicos expected him to wait until January to make the decision that is expected of all Joe Kennedy's sons.

It was "personal" not "political," he emphasized. If he had been thinking only politically, the decision would have been different. He could have been nominated, could have beaten Ronald Reagan.

His children couldn't face another campaign, and he is going through a "painful" divorce. No one who had watched the three of them trailing glumly through town halls in Iowa and New Hampshire could doubt the authenticity of their aversion.

He may support another candidate. He is, he hinted, going to hold them to severe liberal standards.

Former vice president Fritz Mondale is at the head of the line, and might seem his likeliest heir.

But some Democrats can't see Mondale, because Jimmy Carter gets in their way. Others say Mondale is Mondale's problem, being a politician of preternatural caution, whose ability to ignite the country is in doubt.

"I don't know anyone who would walk through a wall for him," said one mourning Democrat.

"Mondale had to rethink all his positions," said Rep. James Shannon of Massachusetts, a Kennedy man. "Kennedy never did. He went against the grain of the country."

And that may be why yesterday, in the huge backwash created by the withdrawal, the name of Morris Udall was heard. The lanky, witty westerner has told friends in the House that he might like to try again.

Said one liberal Democrat, "Mo could lay a greater claim than Mondale to Kennedy's constituency."

Sen. John Glenn of Ohio was mentioned, too. He has, an ardent Kennedyite explained, "no Carter baggage -- he's Mr. Clean and he's been to the moon."

But Glenn is a hawk on defense spending, which Kennedy is not. He is a tiger against nuclear proliferation, but lukewarm on the nuclear freeze, which Kennedy led.

So Kennedy is in no hurry to endorse anyone else. And his followers will await his word. Kennedys have always had people who would walk through walls for them.