LAS VEGAS, APRIL 6 -- Never, ever was this supposed to happen. Sugar Ray Leonard danced, bounced and chattered his way to a history-making middleweight championship tonight, defeating Marvelous Marvin Hagler by split decision in the back yard of Caesars Palace. Call it controversial, call it arguable, but it was the richest, gaudiest prizefight in history, and will also be remembered as one of the most glorious.

Leonard, the slim-shouldered corporate boxer who resigned the 147-pound welterweight title five years ago, was attempting to make improbable history by claiming the World Boxing Council 160-pound championship from the brutish titleholder of nearly seven years standing.

And, at the end, with both men having stayed on their feet throughout the 12 hard-fought rounds, the decision was disputable. There was a wide variation in scoring on the three judges' cards and in the judgment of the media -- the Associated Press made Hagler the winner by a comfortable margin, United Press International saw Leonard as the victor -- and Hagler claimed he was robbed. {Fight details on Page C1.}

Leonard said, "I'm not thinking about a rematch. I'm just going to enjoy the accomplishment."

Hagler said he felt he won the fight. "It's all politics," he said. "It puts a terrible feeling in my mouth for boxing. It puts a bitter taste in my mouth . . . I feel in my heart I'm still the champion."

No long-layoff comeback champion had ever attempted to return in a higher weight class, and the 30-year-old Leonard, after just one mediocre fight in the last five years as a welterweight, did not seem likely to make the transition to 158-pound middleweight (Hagler outweighed him by just a half-pound) successfully. More than that, no champion had ever successfully returned to the ring following an injury as devastating as Leonard's partially detached retina in his left eye suffered in 1982.

"Everyone called it an impossible task," Leonard said.

But in choosing to fight again, Leonard may have proved that there is a compulsive gambler in everyone. After those five mostly retired years of watching Hagler collect glory and winnings that might have been his, Leonard decided to leave his lovely home in Potomac, Md., and return to the city where time is ignored, and where you can get a $2 plate special and a free drink while you take your chance.

He collected an $11 million guarantee for the fight to $11.75 million for Hagler, but that wasn't really the point at all, for Leonard already was a millionaire.

"This fight meant the world to me," Leonard said. "I want to extend my congratulations to Marvin Hagler for giving me the opportunity to make history."

If there was something vaguely troubling about Leonard's decision to fight again, it was because he had been thought of as a boxer of intelligence who had beaten the odds and was able to walk away from the table, unlike the rest of us, when he chose to retire in 1982 because of the injury. But there was also something extremely compelling in the idea of one grand shot, and Leonard made the most of it tonight.

Leonard entered the ring in a short white Elvis Presley-style coat, and danced around the ring. Hagler strode in with his black cloak and theme song, "War," while Caesars helped by setting off fireworks in the shape of an American flag. When the fight began, Leonard shuffle-stepped, dodged and danced away from Hagler, and in the 12th and final round he twice raised his gloves to the crowd, motioning the people to their feet while holding Hagler at bay.

It was the performance of a consummate showman, and equal to those by some of the celebrities in the crowd, which included Frank Sinatra, the Pointer Sisters and Bo Derek, among others.

"My strategy was to stick and move, hit and run, taunt him, intimidate him, and frustrate him," Leonard said.

"Ray Leonard didn't want to say, 'What if?' " his attorney/adviser Mike Trainer said before the fight. "What if is a terrible thing to live with."

Hagler had waited in vain for Leonard to declare a challenge against him as a middleweight in 1982, when both were in their superb primes. He was present at Leonard's invitation for a Baltimore gala that turned into a shocker when Leonard decided to announce his retirement. It was a decision that broke Hagler's heart as much as Leonard's.

This time, neither could endure another fight that might have been.

"Other fights would not have been important to me," Leonard has said.

"This is everything I've ever worked for in my life," Hagler said.

An element of bitterness made the prefight chatter more than just words. There was, of course, the usual banter, like Leonard's trainer Angelo Dundee calling Hagler "a man of all ages," and Hagler's trainer Pat Petronelli producing a birth certificate stating Hagler is indeed 32.

"He's an old man," Petronelli said wryly. "I wanted to give the Leonard camp a shot in the arm. His legs are gone."

Then there was Thomas Hearns, the odd man out tonight, who was beaten so decisively by Hagler in three rounds here in 1985, in what has become one of the most memorable bouts in history. Not to say that it made much less memorable the Leonard-Hearns battle of 1981, also in Las Vegas, in which Leonard unified the welterweight championship by scoring a 14th-round knockout. Those fights were replayed incessantly on big screens all over town this week, pursuing Hearns as he lobbied for a rematch with the winner, and when asked whom he liked in the fight, Hearns replied vehemently, "I don't like either of them."

But if there was true dislike, Hagler had it for Leonard. He was pursued by the memory of making pocket change on undercards while Leonard was making thousands and occasionally millions, and he waited all his career for a chance for a payback.

Leonard was the adored Olympic gold medal champion of 1976, whereas Hagler always has been perceived as the surly, ill-tempered fighter who compared himself the other day to a pit bull, German shepherd and Doberman all in one.

"They come from different places," Top Rank Inc. promoter Bob Arum said.

"It's as though Leonard went to Harvard Business School, got a middle management position, and became chairman of the board. Hagler became chairman of the board too, but he had to start on the assembly line."

The fight didn't end with Leonard's victory. Rival promoter Don King was so thrilled by the stunning upset that he attempted to climb into the ring to congratulate Leonard. Arum, who promoted this fight, leaped on King's back and dragged him out of the ring by his coat. The two began to scuffle when security guards separated them.

Arum's version was this: "King tried to climb into the ring, and I held him down by the seat of his pants. He said to me, 'I'll fight you, sucker, come on down.' I said, 'You're a classless act.' "

For the first time in Las Vegas fight history, other local hotels showed the fight on closed circuit, most of them just across the street from Caesars. A total of 30,000 closed circuit seats were sold in this town alone, in addition to the 15,400 live seats in the outdoor arena in back of the casino. that were sold before the tickets were ever printed.

In the national capital area, both Capital Centre and George Mason University's Patriot Center were sold out, and other area locations were doing brisk business before the fight, with a large last-minute walk-up sale, as well.

Gambling action on the fight was expected to hit an all-time single event high of between $20 million and $25 million in Las Vegas, and Caesars used one of its tennis courts for last-minute betting.