Bruce Finch, working as a high school teachers aide, was on the verge of giving up boxing two years ago. Despite five consecutive victories, he was frustrated and broke, and his career was at a standstill.

Now, he has the opportunity of a lifetime. He is fighting Sugar Ray Leonard, the most popular figure in the sport, for the world welterweight championship here Monday night.

Finch's life style took a dramatic turn in June 1980, when he left his hometown of Milwaukee and made an agreement with an attorney in Las Vegas who had the connections to get him some meaningful fights.

Since joining the management of Keith Galliher, who admits having no previous boxing experience, and casino owner Bernie Dommermuth, Finch has extended his winning streak to 11, knocked out Pepe Dominguez to win the North American Boxing Federation welterweight title and successfully defended it twice.

"I had to bail out of Milwaukee," he said today before a light workout at the MGM Grand Hotel. "I figured sooner or later they were going to arrest me."

Finch said he reached the breaking point when he was involved in an automobile accident.

"It wasn't my fault and there was no big damage to either car," he explained. "But the other guys started talking about a neck injury and wanted to sue me for $2,000, and I didn't have that . . . Then there was this thing with child support. I was giving her money, but she wanted more. That's when I knew I had to get out."

Galliher heard about Finch from Leroy Caldwell, a heavyweight from Milwaukee who was in Las Vegas. The attorney invited Finch out for "an audition," then signed him for his corporation, Punch Your Lights Out Inc.

In his two title defenses, Finch knocked out Dominguez in the second round and Sonny Perez in the third. Finch was scheduled to fight Milton McCrory on the Leonard-Thomas Hearns card last September, but the bout was canceled when McCrory injured his hand.

Still, the 27-year-old Finch admits he was surprised to be matched with Leonard.

"I guess I'm the last of the welterweights," he said, smiling from underneath the baseball hat pulled down over the tips of his ears. "I never thought I could fight Leonard, but I'm not afraid. I'm not nervous one bit."

This is quite a leap from the club fights in Milwaukee, and Finch probably will make more money Monday night ($300,000) than he has in his seven-year career.

Finch's only losses in 34 professional fights were fifth-round knockouts by Pete Ranzany and Larry Bonds in 1977, and a second-round knockout by Hearns in 1978.

Between fights in Milwaukee, Finch worked for 4 1/2 years as what he called "a teachers aide," first at Parkman Junior High and later at John Marshall High School in Milwaukee.

He didn't have any classes. "I was a hall monitor, you know, kept the kids out of the hall," he said. "I didn't mind working, and I wasn't making any money fighting."

Finch said the biggest check he got for a fight was $3,000. He was a winner, but a typical club fighter, and anonymous even in his hometown.

"I can't remember the name of the gym where I fought, but it didn't hold more than 500," he said. "I had 40 amateur fights (35-5) before I turned pro.

"All I was doing was collecting trophies. I told myself I was getting too many black eyes and wasn't even get paid for it. I thought, if I'm going to get black eyes, I might as well get some money, too."

One amateur match was particularly memorable for Finch, although his opponent says he has only a faint recollection.

"I fought Leonard as a amateur," he said, proudly. "I think it was the semifinals of an AAU tournament in Boston, in '73. He beat me on a decision. He was the better man, but I'm a much better fighter than I was as an amateur."

Asked whether he were awed or intimidated about fighting Leonard on national television (Home Box Office), Finch shrugged.

"I don't let any of that bother me," he said. "I've been hearing that stuff all my life, not just from fighters, but from other people. Maybe they don't believe in me, but I have confidence."

And even if Finch walks away a loser Monday night, for a man who was a teachers aide three years ago, just stepping into the ring with Sugar Ray Leonard is a victory of sorts.