It might be difficult to persuade the 130,000 people who will be screaming here at 5:38 p.m. Saturday, but this is a bad Kentucky Derby field.

The 108th running of America's most famous race (WJLA-TV-7) was hurt by the defection of three of the best 3-year-olds in the country. It has been tarnished further by a stampede of undeniably mediocre colts to Churchill Downs. Of the 20 horses whose names were dropped into the entry box Friday, at least half have no plausible chance to win and don't really belong in the race.

What makes the Derby unique is that the low quality of the field almost doesn't matter, because whoever wins it will earn a measure of immortality regardless. When visitors here see legendary names like Hindoo and Twenty Grand on the walls of the clubhouse, on the souvenir T-shirts and mint-julep glasses, nobody stops to wonder whether these horses happened to get lucky and beat weak fields in a year when the favorites were injured.

Years from now, nobody will remember that El Baba or Muttering or Air Forbes Won captured the 1982 Kentucky Derby because he got very lucky. These colts already have benefited from the illness that knocked Timely Writer out of the race, the injury that ended Hostage's career and the judgment of trainer Henry Clark, who chose not to come here with Linkage. Now all these colts need is some racing luck on Saturday afternoon.

The presence of so much deadwood in the field is not irrelevant to the outcome of the Derby, because even bad half-ton animals can take up a lot of room. The large field has to hurt the chances of plodders like Royal Roberto and Gato del Sol, who will have to wend their way through the heavy traffic.

But the three favorites in the race all have the speed and quickness to avoid trouble, capable jockeys to help them and decent post positions. Bettors trying to choose among them may have trouble, because this Derby is a true intersectional battle. Air Forbes Won has done all his racing in New York; Muttering has never before run outside of California; El Baba has spent his whole career in the midlands.

El Baba has lost only two of his 10 races, finishing second to Linkage in the Louisiana Derby and second to Hostage in the Arkansas Derby. "He couldn't be more consistent," said trainer Dewey Smith. "You couldn't ask for more in a horse."

But the pedigree pundits wonder if El Baba might ask for some genes that would make him a better prospect at 1 1/4 miles. The progeny of his sire, Raja Baba, usually don't want to go much farther than a mile. "I've been told that," Smith said, "but this horse always has looked like he has stamina. And he's got the determination to win."

While El Baba's overall record is exemplary, Air Forbes Won's is perfect. He is the first undefeated horse to come to the Kentucky Derby since Seattle Slew in 1977. He has the best jockey in America, Angel Cordero Jr. And yet he has generated surprisingly little enthusiasm here, because his final prep for the Derby--a slow, photo-finish victory in the Wood Memorial Stakes at Aqueduct--was so unimpressive.

But his trainer, Frank LaBoccetta, thinks the lightly raced colt has not yet begun to show the full extent of his ability, and thinks the Derby is the place where he can do it. He believes Air Forbes Won is especially well-equipped to deal with a 20-horse field.

"If he gets into trouble," LaBoccetta said, "he's got the acceleration to get out of it. He'll accelerate any time the jockey asks. He'll give you three runs in the same race."

Muttering established himself as the best 3-year-old on the West Coast when he won the Santa Anita Derby even though, trainer Wayne Lukas said, "We hadn't really tightened the screws yet."

Now he has tightened them. He is bringing Muttering into the Derby off a four-week absence from competition, training him the way he trained Codex before his explosive victory in the 1980 Preakness. "I've been through this battle before," Lukas said, "and I know I'm not guessing. Muttering is a very legitimate horse."

Of the other 17 horses competing for the record Derby purse of $527,600, only a few can be considered contenders. Royal Roberto has been prepared masterfully for this race by trainer Jimmy Iselin, and he is a colt who wants to run a mile and one-quarter.

Bold Style demonstrated his quality when he finished only a length behind El Baba in the Arkansas Derby. Star Gallant, with Bill Shoemaker riding, ought to lead the Derby for a mile, although he has never demonstrated that he wants to go much farther. Cassaleria has the distinction of being the best one-eyed 3-year-old in America. That is one of the few unqualified superlatives that can be applied to any of the horses in the field.

BALTIMORE, April 30--Marshua's Dancer saved ground throughout today's feature race at Pimlico and won by a head over Sailing Hour, a Henry Clark-trained filly who reared at the start but made up ground with every stride.

Marshua's Dancer was ridden by Mark Slaven and ran the six furlongs over a fast racing strip in 1:11 1/5. She paid $13.60 and capped a 1-5 exacta worth $103.20.