The fighters who intrigue the American boxing public above all others -- Sugar Ray Leonard and Thomas Hearns -- will perform on the same card Thursday night at the Astrodome. Unfortunately, they don't fight each other.

Each man will be fighting for a title, Leonard challenging champion Ayub Kalule for the WBA junior middleweight championship at approximately 11 p.m. (EDT), Hearns defending his WBA welterweight title against Pablo Baez at 10:15. The fights will be telecast via closed circuit, at Capital Centre in the Washington, D.C., area.

The stakes for Leonard, the WBC welterweight champion, and Hearns, undefeated in 31 fights, are considerably higher than mere championships. There is talk that the Hearns-Leonard match to unify the welterweight title on Sept. 16 will be a $20 million affair. A loss here by either man would necessitate a renegotiation between the two fighters.

Leonard would seem to be taking the greater risk, because Kalule, 27, has not lost in 36 fights, has never been knocked off his feet as an amateur or a professional and insists he will go back to Denmark with his championship intact.

He is mostly a straight-up, charge-ahead fighter who inflicts heavy damage to the body. He has 18 knockouts, though his last three fights went the distance. He is not considered a powerful puncher, simply a relentless one. He also is a southpaw, though his right hand is considered his strongest weapon.

"Pressure is what he does best," said Angelo Dundee, one of Leonard's trainers. "He tries to dictate the fight. Physically, he reminds me a lot of Emile Griffith. The guy's got quick hands and a good cheek. I've seen him get hit, and he doesn't flinch. I never take nothing for granted. Who knows, he's liable to come up with the greatest fight of his life. But my guy's ready, too."

Leonard, 25, with a 29-1 record, is taking a step up in class from 147 to 154 pounds. He will probably give away five to six pounds on fight night, though he is far faster on his feet and has a better punch than Kalule. Adds Dundee: "In this fight, Ray's gonna be the banger."

"I can envision a fight that is going to be fought at a very fast rate," Dundee said. "I also see Ray jumping at Kalule, jumping in his face, seeing surprise on his face and him saying: 'Wow, this guy can really hit.'"

Janks Morton, Leonard's full-time trainer, predicted today that "with the condition Ray is in now, I see no way Kalule will last eight rounds. You're going to see the greatness come out in Ray now."

Morton also insists that Leonard has not pooh-poohed Kalule, that he is not looking ahead to hearns, that he has prepared more diligently for this fight than for either of the confrontations with Roberto Duran.

"The Hearns fight doesn't cross his mind," Morton said. "We never call Hearns' name at all. It's Kalule all the way. Kalule really is an unknown, but that doesn't mean the guy can't fight. . . I've been looking at him. He's sort of mechanical and throws a lot of punches. He's also got a big face and head, a nice target. We're well prepared. We had 90 rounds of boxing for six weeks for this fight. Against Duran the last time, we had 75 rounds and four weeks."

Leonard already has predicted the fight will go no longer than 10 rounds. The money, $2.5 million for Leonard, $150,000 for Kalule, is not a motivating factor, he says. A second title is. And next year, maybe a third, because Leonard eventually will fight in the middleweight (160 pound) division.

"Ray isn't fighting for the money anymore," said Mike Trainer, his attorney. "I just think he'd like to establish himself as the best fighter pound-for-pound who ever fought."

Hearns would like that distinction himself, though he will hardly get into that class by fighting the likes of Baez, a 23-year-old native of the Dominican Republic who lives in Los Angeles. Baez, who some people have nicknamed Joan, is 14-8-2 and ranked ninth by the WBA, though he appeared in the rankings just a few weeks after he signed for this fight.

Hearns, 22, will be making his third title defense and going for his 30th knockout, which most people assume will occur in the first five rounds. He will earn $500,000 for his work, with Baez getting $50,000, easily the largest payday of his life.

Emanuel Steward, Hearns' manager, insisted today that Baez has improved since he got new management last year, and dismisses talk about Baez being in the Stiff Hall of Fame.

"(Pipino) Cuevas was supposed to be the toughest fight we ever had; he was supposed to beat up the little kid from Detroit, and we knocked him out in the second round," Steward said. "Randy Shields was supposed to be an easy fight, and he may have been the toughest. So you know what prefight publicity means."

Bob Arum, who is promoting this fight, also knows about publicity. Today, he said he planned to file suit to hold up Leonard's purse Thursday because he was upset over all the talk about Hearns vs. Leonard, and not enough about his own promotion.

He was particularly angry with Trainer for looking ahead in a story published in the Houston Post. "You don't talk and say another fight is made until this fight is over," Arum said. "This is insidious, designed to destroy the promotion."

"Oh, I think we're going to show up," said Trainer, who has letters of credit in the bank that guarantee payment as long as the fight goes on. "He's established Bob Arum's common law on fight promotion . . . whatever he cares to say. I don't pay any attention. It makes a nice headline."

And headlines help sell tickets, which are not very much in demand here for this production. Only 15,000 had been sold by today, and the closed circuit sites, with some exceptions, are not reporting great business.

It may be Thomas Hearns and Sugar Ray Leonard, the fighters. But not The Fight.