In 1973, as he was being prepared for the Belmont Stakes, Secretariat was becoming a national hero. He was so celebrated because he was pursuing the most elusive of goals. He was trying to win the Triple Crown, something that had not been done for a quarter-century and was begining to seem impossible.

On Saturday, Spectacular Bid will attempt to accomplish the same feat, but his quest is not surrounded by the mystique that Secretariat's was. If he wins the Belmont, Spectacular Bid will be the fourth in seven years. The impossible has become commonplace.

This might be an accident of history. There was a similar cluster of Triple Crown winners in the 1940s, when four horses in eight years won the Kentucky Derby, the Preakness and the Belmont. But racing people sense that the nature of Triple Crown competition has changed, that it has become easier for one horse to sweep the series, and they have formulated many theories to explain the phenomenon. (This week's Sports Illustrated suggests two of the possibilities: Horses may be getting better. Or they may be getting worse.)

The Triple Crown used to be so hard to win because a horse had to maintain peak form over a long period while encountering a succession of new, fresh challengers.

Forward Pass lost the Triple Crown in 1968 because a fresh, tough Stage Door Johnny was waiting for him at Belmont. Riva Ridge missed in 1972 when Bee Bee woke up and ran the one great race of his life in the Preakness. Horses trying to win the Triple Crown had to cope with quantity as well as quality. Kauai King (1966) and Canonero II (1971) were faced by 10 and 12 rivals, respectively, when they ran in the Belmont.

The Triple Crown races, and the Belmont in particular, are not as competitive as they used to be. Secretariat had only one serious rival, Sham, in his Triple Crown series. Affirmed had to beat only one dangerous horse, Alydar, to win it last year. Seattle Slew had so little opposition he won the Triple Crown almost by default. Spectacular Bid has not faced any dangerous challengers, either.

What has happened to all the good horses? They haven't ceased to exist. They have found new places to run.

A decade ago, a man with a talented 3-year-old who wanted to go after the big money had no option but to aim for the Triple Crown races. But as racing has proliferated across the country, so have opportunities for 3-year-olds.

West Coast 3-year-olds get to run for such large purses early in the year that the Kentucky Derby comes almost as an anticlimax. The California horses may be burned out by the time they get to the Triple Crown races. This year's West Coast champ, Flying Paster, had certainly peaked by the time he met Spectucular Bid.

During the spring and summer, there is an abundance of lucrative races for 3-year-olds. A trainer who wanted to avoid Spectacular Bid in the Belmont Stakes could set his sights instead on the $100,000 Pennsylvania Derby, $150,000 Ohio Derby, $100,000 American Derby, $100,000 Omaha Gold Cup and many other races. These events have been siphoning off the Bee Bee Bees and the Stage Door Johnnys who have sprung Triple Crown upsets in the past.

The existence of these alternatives to the Triple Crown will make Spectacular Bid's task much easier Saturday. There is one 3-year-old in America who could conceivably beat him, but that animal probably will not be entered in the Belmont.

His name is Coastal. After winning the first two starts of the season in allowance company, Coastal ran in the Peter Pan Stakes, which is designed as a prep for the Belmont. His performance was impressive. He ran away from his nondescript opposition by 13 lengths and covered the mile and one eighth in 1:47 over a racing strip that was not exceptionally fast. Students of speed handicapping would give his victory a bigger figure than Spectacular Bid earned in the Derby or the Preakness.

Coastal lacks Spectacular Bid's seasoning - he has never raced around two turns - but if there were no alternative trainer David Whiteley might try to capitalize on his sharp form and take a shot in the Belmont. But there is no need to. Whiteley probably will send his late-blooming colt to the Ohio Derby, and let Spectacular Bid win the Belmont almost by default.