Of all the people at Churchill Downs next Saturday, none will be gambling so boldly as David Cross. The 48-year-old trainer has virtually staked his career on the prospects of a colt named Sunny's Halo. And he has prepared him for the Kentucky Derby in a manner that defies all the conventional wisdom of his profession.

A year ago, when Sunny's Halo started showing signs of exceptional ability, Cross was training a 30-horse stable in Canada and California. But since then he has devoted his time and effort so single-mindedly to this one animal that his owners have deserted him; he has only two other horses left. This, he thought, has been a sacrifice worth making for the horse of a lifetime, the opportunity of a lifetime.

Cross was virtually raised at a now defunct track, Willis Park in British Columbia. "The first thing I remember is having a pitchfork in my hand," he says. He rode for a couple of years, then started training in 1957, but he never had any grand dreams until Sunny's Halo won several stakes in Canada.

The trainer took him to major races at Belmont, Saratoga, Laurel and the Meadowlands--prompting the first defections by his other owners who wanted him paying more attention to their horses--but these ventures were unsuccessful.

Sunny's Halo had wrenched an ankle in midseason and Cross "was so concerned about that, that he had a shin problem I let get away from me by neglect. I shouldn't have run him in some of those races. X-rays showed he had a stress fracture."

This winter Cross took Sunny's Halo to California, where the horse spent several days a week in a swimming pool so he could get fit without putting too much pressure on his shin. As he did, Cross started formulating his plans.

The proved method to prepare for the Derby is to race in Florida or California and to give a horse at least three or four prep races. Cross rejected Florida ("a swamp") and California ("the Santa Anita track scared me to death") and took Sunny's Halo to Arkansas: "I don't think there's a better place for a horse to bloom." He didn't care that the Arkansas Derby has never produced a Kentucky Derby winner.

He decided, also, that two races before the Kentucky Derby would be sufficient--even though no horse in decades has won here with so little preparation.

So far nobody can second-guess what Cross has done. Sunny's Halo won the Rebel Handicap by three lengths, then led all the way to capture the $250,000 Arkansas Derby in sensationally fast time. He won with conspicuous ease, and his victory looked even better when the rival he defeated by four lengths, Caveat, came back to win Saturday's Derby Trial at Churchill Downs.

This morning Sunny's Halo completed preparations for the Derby as he worked a mile in 1:41 2/5 in a driving rainstorm. Now Cross can only hope he is fit to go 1 1/4 miles.

If Sunny's Halo wins the Derby, he might be worth as much as $15 million, and Cross said owner D.J. Foster, a Toronto stockbroker, has been negotiating to sell a half-interest in the colt.

If Sunny's Halo even runs respectably, Cross and Foster will have a multimillion-dollar property. But if the horse runs poorly, Cross will be a man with a three-horse stable who trained a contender for the Kentucky Derby all wrong.

"There's a lot riding for you on this race," a journalist suggested this morning. "You'd better believe it," David Cross said.