He has won more money than any thoroughbred in history. He thrilled a nation with his exploits in the Triple Crown series. He has been a champion in all three years of his racing career.

And yet, for all of his lofty achievements, Affirmed has not yet proved himself a great horse. What he does in the next two months, before he is retired to stud, will largely determine his standing in sports history.

Such reservations about the colt's past accomplishments may seem absurd to many racing fans, not to mention the breeders who have already established Affirmed as a $14.4 million stallion. But a critical review of his career suggests that his record is less formidable than his reputation.

Affirmed became a hero in the spring of 1978 when he beat Alydar in three dramatic battles and captured the Triple Crown. Even the most casual racing fans could appreciate the virtues he displayed in these confrontations. He was tough. He was game. When Alydar challenged him in the stretch in the Preakness and the Belmont Stakes, Affirmed refused to let him go by.

The memory of those stretch duels is still vivid: some of the circumstances surrounding them have been quickly forgotten. In the Preakness and the Belmont, the deck was stacked in favor of Affirmed. In both races, he was the only speed horse in the field. He could go to the lead and set his own leisurely pace, forcing Alydar to alter his usual stretch-running style and take up the chase prematurely.

In almost any event that shapes up as the Affirmed-Alydar races did, with a single speed horse being challenged by a stretch runner, the horse in Affirmed's position will win. (It happened again this summer at Saratoga, when Affirmed's stablemate, It's In The Air, upset Davona Dale in the Alabama Stakes.) Affirmed won those Triple Crown races in the first half mile, not in those thrilling stretch battles.

What would happen if Affirmed had to face a tough opponent under conditions that were not so favorable? That question was answered in the fall when he faced the previous year's Triple Crown winner, Seattle Slew. It was no contest. Seattle Slew ran away from Affirmed and beat him by three decisive lengths in the Marlboro Cup.

Affirmed's apologists point out that the colt suffered these defeats at the end of a long, hard season, while Seattle Slew was fresh at the time. That may be true, but since those losses Affirmed has done nothing to demonstrate that they were flukes.

In 1979, he has added $800,000 to his bankroll but little to his reputation. Trainer Laz Barrera kept him on the West Coast, where Affirmed lost his first two races and then won his next four, against competition of dubious quality. When he captured the $500,000 Hollywood Gold Cup, a horse named Text ran well against him and finished third. Two weeks ago, in the Marlboro Cup, Text was dismissed as 44 to 1 and finished next to last as Specatular Bid won with ease. Affirmed's victories over Text and the other California horses did not prove much.

But soon Affirmed will have the chance to prove everything. Saturday, he begins his fall campaign in the $175,000 Woodward Stakes at Belmont Park. His opposition will not be overwhelming, but at least Coastal will provide a better foil for him than his West Coast rivals.

Two weeks later, Affirmed will probably have his decisive showdown with Specatular Bid in the Jockey Club Gold Cup. And then, if his owner and trainer are venturesome, the colt could try to beat this country's and Europe's best grass runners at their own game, in the Turf Cup at Belmont or the Washington, D.C. International at Laurel. Only if he passes some of these tests will Affirmed deserve to take a place alongside the past greats of racing history.