First, it was Marvin Hagler installed as the 7 to 5 favorite in Las Vegas' betting parlors. Then in came the Hearns money and it was Thomas Hearns the 6 to 5 choice to take Hagler's title. Today, after more vacillating, it is an even money, pick-'em fight.

Where are the strong opinions, and what kind of weaseling is this? Bob Martin, a top analyst of the Las Vegas money market and an old hand at construing the moods, puts his own interpretation on the betting patterns of the fight: "They go to bed at night thinking Hagler and wake up in the morning thinking Hearns."

Similarly, it is a pick-'em affair for those who care to listen to the confident profundities emanating from each of the two camps.

From the Hagler people: Hearns can't stay away from the champion and if he is impudent enough to try to slug it out with us, Hearns is a goner, quick. Hearns will never know which side our man is attacking from, and who's afraid of Hearns' right hand?

From the Hearns camp: Hagler has been around for too many years, and we'll locate his weaknesses, quick. We're not worried about Hagler's body attack, because we don't aim to let him inside. Our man will use the whole ring, and Hagler will never know where the big right hand is coming from.

These, it must be noted, are mainly the mouthings of the brave trainers, managers and camp followers of the two principals. But, as a boxing-wise old-timer once said, "All the decisions will be made and the fighting will be done by those two men on those two stools."

It will be a curiosity fight, with a bundle of questions waiting for answers when the two men step into the ring Monday night. First, which one will dare to throw the first punch, always a fascinating suspense? Then, more importantly, who will dictate the fight, Hagler with his walk-in fury or Hearns with his movements, his superior boxing ability plus his right-hand grenade?

More answers will be revealed with each passing minute, but it may be well to note that Hagler-Hearns is an odd mixture anyway. Individually, they do not have great charisma and could be bores, and not much at the box office. But thrown together, they are a volatile matchup with their contrasting styles, their raging hate for each another, and their .962 batting average against the 105 men they have fought. Mr. Muscles vs. Mr. Bomb.

It is not a case of Hearns, as a former 147- and 154-pound class champion, taking on Hagler as a biological blowup who suckled on egg milkshakes to reach 160 pounds. On his 6 feet 1, Hearns wears his new poundage easily. He's a valid middleweight. His important weight is where it belongs, in a V-shaped expanse toward the shoulders where the punching power resides. He is moving up from the welterweights, but Hearns won't be the little man in the ring against Hagler.

In his own smug way, Hagler has viewed Hearns' advantage in height and reach: "Tall men make mistakes," he said, leaving little doubt he is counting on such. That has been Hagler's habit in all of his fights, to probe for the guy's vulnerability and then exploit it with the fists that have won 60 fights, 50 by knockouts.

Both men are not without blemishes in their past. Hagler dearly would like to forget that an aging Roberto Duran, who later was to be stopped in two by Hearns, carried him to 15 rounds and was ahead after 13. Also, that Wilford Scypion rocked him with a right hand in the third round before Hagler rocked Scypion to sleep in the fourth. And John Roldan put Hagler down, although the Hagler people say it was a slip.

Similarly, Hearns has had some forgettable fights, including his 14th-round knockout by Sugar Ray Leonard, whom he let get off the hook. That night Leonard put Hearns away with a 25-punch barrage on the ropes. Recently, a couple of other opponents of little note went the route with Hearns.

Hagler and Hearns were supposed to meet in 1982, but Hearns had a damaged finger and it has proved profitable to both. In 1982, they were to fight for $3 million. Now they'll split $11 million guarantees, and more from closed circuit.

There was a hitch in this fight, too. After promoter Bob Arum thought he had the match made with a $10 million guarantee, the fighters upped their demands to $11 million. With the fight slipping away, Arum solved the problem by getting Caesars Palace to put up half of the extra million, he the other half.

Hagler recently helped to pump up interest in this one by knocking out Syrian middleweight Mustafa Hamsho in three at Madison Square Garden. Arum, a former Talmudic scholar, had refused the request of Syrians to play the Syrian national anthem as a greeting to Hamsho. Later, according to Arum, "Somebody bribed somebody, and they slipped a Syrian anthem cassette into the record player just as Hamsho came down the aisle."

One of the preliminaries to the fight already has been held. Ex-champ Jake La Motta, on Arum's payroll as a touring publicist, took to wife bride No. 6 in a ritual at Maxim's Hotel with a philosophic La Motta explaining that he intended to continue until he got the right one.