When the horses come onto the track for the Kentucky Derby, the band strikes up "My Old Kentucky Home" and lumps form in the throats of some 150,000 people at Churchill Downs, the biggest lump of all ought to be felt by young Ron Franlkin.

Rarely has so much of the pressure of the Derby been funneled toward a single individual as it has toward the 19-year-old jockey for Spectacular Bid.

To many observers, the great issue of the 105th Derby is not the East vs. West confrontation of Spectacular Bid and Flying Paster. Rather, it is the question: Will Franklin mess it up?

Never has a Derby favorite been ridden by a jockey more likely to mess it up.Franlkin lacks the experience of his rivals: he was 4 1/2 months old when Flying Paster's jockey, Don Pierce was riding in his first Derby. And he lacks the ability: His panicky, ill-judged performance in the Florida Derby this winter nearly cost him the mount on Spectacular Bid.

Franklin's credentials would be unimportant if Spectacular Bid Figured to lead all the way, or if he had drawn an outside post position where he could stay out of trouble. But the Derby will be fraught with pitfalls for him Saturday.

Spectacular Bid drew the No. 3 post position in the 10-horse field. Since he has not been coming out of the gate sharply in his recent races, there is a great likelihood that horses outside him will break faster and angle toward the rail, boxing him in.

This possibility is especially ominous because one of the jockeys outside him is Angel Cordero Jr., whom Franklin and trainer Bud Delp accused of trying to hurt Spectacular Bid in the Florida Derby.

Their accusations had racial overtones, and if Cordero has a chance to retaliate by causing trouble for Franklin, he surely will do so.

Such prerace worries had seemed unlikely in March when Delp and owner Harry Meyerhoff decided to stick with Franklin in the wake of the Florida Derby Debacle.

At the time, their horse looked so good that the choice of a jockey seemed academic. And there was no sign of serious competition for him anywhere in the nation.

Now the situation has changed. Although he has extended his winning streak to 10 races in a row, Spectacular Bid has not looked so awesome recently. The time of his victory in the Blue Grass Stakes last week was so slow that it triggered suspicions the horse might have peaked too soon.

And, meanwhile, a threat to Spectacular Bid began developing in the West. Flying Paster was the best 2- year-old in California last year, but his credentials were tarnished by a loss in a minor stakes race in March.

Since then, however, he has run away with the Santa Anita Derby by 6 1/2 lengths and captured the Hollywood Derby by 10 lengths in stakes-record time. Californians have been comparing him favorably with Affirmed, who won the same races last year before taking the Derby.

If Spectacular Bid may be on the downgrade, Flying Plaster seems to be coming to the Derby at the peak of his form. But his chances could be seriously harmed by the weather, and the track condition.

This has been a rainy Derby week, and today the Churchill Downs track was a sea of slop. It seems unlikely that the track will dry out enought to be fast by the 5:38 p.m. post time. Flying Paster has done all his racing on hard, fast California tracks, and trainer Gordon Campbell said his colt has not even done much training in the mud.

Spectacular Bid ran without success in mud as a 2-year-old but Delp attributed that to the colt's lack of experience.

Although the Derby is widely seen as a two-horse race, with Spectacular Bid likely to be favored at 4 to 5 and Flying Paster at 8 to 5, four other entrants have legitimate credentials and a shot at the $304,900 purse.

General Assembly was the second-ranked 2-year-old in the nation last year, but has been an erratic performer this season. He did run exceptionally well to capture the Gotham Stakes at Aqueduct, and his sensational five-furlong workout in 57 2/5 seconds this week suggests he is ready to run a big race again. He can win the Derby.

Screen King earned a trip to Churchill Downs after fininshing strongly in the Wood Memorial Stakes at Aqueduct and losing by a nose. But that was a weak field and a slowly run race. Screen King Bears watching on Saturday mainly because of the presence of Cordero in the saddle.

Golden Act won the Louisiana and Arkanasas derbies with a strong stretch run, and he will be suited by the 1 1/4-mile distance. But the caliber of opposition he beat in his previous victories does not suggest he can cope with the likes of Spectacular Bid.

The others in the field are Shamgo, King Celebrity, Sir Ivor Again (a stablemate of General Assembly) and Great Redeemer. The latter is a hapless maiden whose owner, Dr. J. A. (Randy) Mohamed, has written a book on thoroughhbred breeding and advertised it under headline, "Spectacular Bid will not win the Kentucky Derby."

Lot O'Gold may be 50 to 1 Saturday but he is a much better colt than those odds indicate. He has been soundly beaten by Spectacular Bid four times this season, but his losing efforts were better races than the victories by some of the other Derby contenders. Still, there is no basis for agreeing with trainer Smiley Adams, who predicted, "I'm going to be putting those roses around his neck Saturday."

Great Redeemer drew post position 2, the stall next to Spectacular Bid, and only needs to make a right-hand turn coming out of the gate to make his owner's prophecy come true. That's another thing for Franklin to worry about Saturday - not that he needs it.