Slew o' Gold has won all of his races this year with such authority that he might seem unbeat- able. When co-owner Mickey Taylor talked the other day about his horse's prospects in Saturday's $3 million Breeders' Cup Classic, he sounded as if he thought the outcome was a foregone conclusion.

That impression is mistaken. Beset by nagging physical problems all year, Slew o' Gold has at no time resembled the horse who was a genuinely great performer in the fall of 1983.

The problems won't go away; a newly developed crack in his right front hoof has made his status uncertain for Saturday's race, although Taylor insists he has an 80 percent chance of running.

But it almost doesn't matter. Even if a veterinarian manages to patch the foot effectively enough to get Slew o' Gold to the starting gate, he is not going to win the most valuable thoroughbred race in history.

In his current form, he is not going to beat Gate Dancer. (And he may not beat Precisionist, either.)

The extent to which Slew o' Gold has been bothered by physical problems may not have been obvious to casual viewers who watched him sweep through New York's fall championship series.

But he was winning comfortably only because his opposition was so weak.

The times of the races disclose just how much he has declined since he narrowly missed sweeping the same series in 1983.

Slew o' Gold was capable of beating first-class horses last fall, but this year he has been lucky to have run against the likes of Canadian Factor, Bounding Basque and Shifty Sheik. His luck has run out, because two entrants in the Breeders' Cup Classic are very formidable horses.

Not many Easterners are familiar with Precisionist, but the 3-year-old has dazzled the most astute handicappers in California. He led all the way to win the Hollywood Derby by 10 lengths.

In the Del Mar Handicap, he set a seemingly suicidal pace -- running the first half mile in 45 seconds flat -- and still managed to win in brilliant time. Then he went to the $500,000 Super Derby at Louisiana Downs, controlled the pace, opened a three-length lead turning for home and was caught at the wire by Gate Dancer. The two horses broke the track record for 1 1/4 miles by more than a second.

Just as Slew o' Gold blossomed at the end of his 3-year-old season, Gate Dancer seems only now to be reaching his peak. Even when competing in the Triple Crown series -- in which he set a track record in the Preakness -- he wasn't a terribly impressive racehorse. He had a lot of bad habits, and he was beating a very ordinary group of 3-year-olds.

But Gate Dancer's performance in Louisiana suggests strongly that he is now the best thoroughbred in America. In the Breeders' Cup, he seems to have plenty of edges. The field is loaded with speed horses -- Desert Wine, Track Barron and Slew o' Gold's pacesetter, Mugatea -- who should subject Precisionist to early pressure and abet a horse with Gate Dancer's stretch-running style.

Moreover, Gate Dancer is permitted to use the drug Lasix, which he seemingly needs in order to do his best. He will be feeling good on Saturday, and Slew o' Gold assuredly won't.