Phil Mahre's chances of becoming the first American to win skiing's World Cup ended today when he broke his left leg in a giant slalom race on the soggy slopes of Whiteface Mountain.

America's finest ski racer and its best hope for a gold medal in the 1980 Winter Olympics never crossed the finish. Two-thirds through his run, he lost control, smashed into a gate and somersaulted to a stop. He threw his gloves down angrily.

"Dammit, I think I broke my leg," he said.

"I was right there," said his coach, Hank Tauber. "Before he even stopped sliding he was saying, 'It's broken.'

"He was in pain," Tauber said, "but the worst pain was the disappointment."

Ingemar Stenmark of Sweden won today's race for his eighth straight victory this season. But the big winner was Switzerland's Peter Luescher, the man Mahre was trying to catch for the World Cup title.

Mahre broke his left leg just above the ankle. By midafternoon he was in bed at Placid Memorial Hospital and Monday morning he will be flown to California for surgery.

Dr. Arthur Ellison, an orthopedic surgeon, said prospects are good for full recovery. He said a pin will be needed to repair the bone.

"Under normal conditions Mahre would be in a cast about two months and should be able to begin normal sports activities again by June or July," Ellison said.

The surgery is to be performed by Dr. Richard Steadman, head orthopedist for the U.S. ski team. Steadman said the tibia had been "broken into more than two pieces."

Mahre was second in the World Cup last year, the highest-ever American finisher. This year he started badly. He and his twin brother Steve, the No. 2 U.S. man, were sent home from Europe for three weeks just before Christmas for what skiers call depression.

He came back to the World Cup circuit on a tear, finishing first, third and second in his last three slalom races.

Then in the giant slalom last month at Are, Sweden, he finished second to the undefeated Stenmark.

Then came Placid.

The 21-year-old American was on home turf. His wife of six months was with him. This is his favorite time of year.

"I'm peaking right now," he said after a practice run.

He started worrying Luescher, who was complaining about painfully stretched knee ligaments.

"Philip is coming after him and Luescher knows it," Tauber said before today's race.

But by yesterday afternoon Luescher was bathing in the almost certain knowledge that he would be the 1979 World Cup champion. He had pulled an early starting number for the giant slalom and raced to the best finish of the day in the first heat.

Then came Mahre's disastrous run.

"It's too bad," said the 23-year-old Swiss racer. "The victory would have been far better if he were there. I would have liked to go to the finish with him."

Luescher wound up third in the giant slalom after Stenmark made a spectacular second run and Hans Enn of Austria finished strongly.

Stenmark opted out of the World Cup title chase this year by not entering any downhill races. He completes in only slaloms and giant slaloms.

But he, too, was disappointed at Mahre's accident.

"It's too bad for the World Cup," said Stenmark, who has won the last three World Cup titles. "He had a good chance to win it. It doesn't matter that I can't win this year. I have won it enough."

For Luescher, the expected victory will be sweet.

"It's the biggest you can do, except for the gold medal in the Olympics," he said.

World Cup points are earned for top finishes in each discipline, with extra points available the last three races of the year. Luescher still faces challenges from Andreas Wenzel of Liechtenstein, Piero Gros of Italy and Leonard Stock of Austria. But Mahre was the closest and his threat is over.

No one knows what caused the crash, but both Tauber and the men's team coach Harald Schoenhaar, said the snow was "grippy." Officials sprinkled ammonium hydrate on the course before the race to harden snow that had grown soft under light rain and temperatures nearing 50.

The "salting" is common practice but it makes for "funny snow," Tauber said.

Mahre was not having a great run.Schoenhaar, who also watched the crash from the sidelines, said Mahre apparently got his ski tips crossed.

"He was hustling," the coach said, "trying to make up time. He had made a couple of minor mistakes up top."

Mahre hit the bottom set of poles in a closed gate and went down instantly.

Within minutes he was being transported down the mountain in a litter. Paula, his wife, sobbed on Tauber's shoulder as the procession passed by. Her husband was wrapped in a blue blanket, his eyes clouded by the pain.

"You can spend a lifetime in a sport like this and never coach a superstar like him," Tauber had said a few days before.

"Sometimes when I'm coaching him I watched him go by at 80 or 85 miles an hour. He's asleep out there. I'm amazed. I just look at him and say, 'What a boy.'"

Mahre had a broken leg, before, but never the left leg. In 1974 he got caught in an avalanche in his home town of White Pass, Wash., and broke his right leg above the ankle when he was pushed against a tree stump.

He reborke that leg, which apparently healed fully, in the same place a few months later in a playground accident.