Seattle Slew came back from the ungrateful dead yesterday to bury Affirmed in the test of Triple Crown winners run as the Marlboro Cup.

The margin of victory was three lengths but it did not begin to tell the story of Slew's powerful performance. He was THE horse, every step of the mile and an eight, riping off fractions of :47 and 1:10 1/5 en route to a final clocking of 1:45 4/5, only two ticks of Secretariat's 1973 track and stakes record.

Affirmed never made it close. The 1978 Kentucky Derby, Preakness-Belmont victor was favored at 1 to 2 by the disappointing crowd of 40.880. He appared to have a decided edge but he had no excuse, holding second place all the way, while finishing five lengths ahead of another 3-year-old, Nasty And Bold, in the field of six.

Seattle Slew's victory in the $300,000 sponsored event was worth $180.000.m It meant much more than that, however, to his $12 million syndication value as a stallion. There were those in thoroughbred racing and breeding industry who had begun to doubt his 1977 achievements, when he emerged as the only undefeated Triple Crown winner in the long history of the sport.

"After your first loss (last June) you fired your trainer. After your second loss (earlier this month) you fired your jockey." Phil Johnson, training of Nasty And Bold, reminded the partially new Siew Crew Thursday, asking, "I want to know what will happen if Seattle Slew loses Saturday, will you give your horse away?"

Doug Peterson replaced the popular Billy Turner as Slew's trainer late last year. Angerl Cordero was named to replace Jean Cruguet as the 4-year-old colt's jockey after the recent defeat in the Meadowlands. Mickey and Karen Taylor and Jim and Sally Hill, the owners, were criticized for every move they made. Then, yesterday, all was forgiven.

"We never gave up on Slew: we were confident, despite all the problems he's had," Mickey Taylor declared. "There was the virus that nearly killed him last winter in Miami, then a leg injury and more recently, show troubles. It was tough getting him ready, but Doug did a fine job."

And Cordero, as usual, rode a great pressure race.

Cordero finally put the slumping Steve Cauthen in his place, after suffering a long string of defeats in attempting to beat Affirmed with lesser runners. Cordero orginally had the mount on Affirmed, only to jump off in favor of Derby Creek Road last summer at Saratoga, never to regain the mount.

"We had it all the way," a proud particularly pleased Cordero boasted on his return to the jockeys' room. Seattle Slew was first by two lengths after a relaxed opening quarter-mile in :24. From there Slew proceeded to run six furlings in 1:09 3/5 with fractions of :23, :23 1/5 and :23 2/5, and still had plenty left with which to finish the final eighth of a mile in :12 1/5.

Affirmed never quit trying, "He broke good . . . and I never took my eye off Seattle Slew, "Cauthen said. "At the top of the stretch I got after Affirmed. He tried his best. He ran his race. It just wasn't good enough, added Cauthen, who is six for 60 at the current Belmont meeting.

Laz Barrera, Affirmed's trainer was critical of Cauthen's sense of pace.

"You just don't go :24, :47," Barrera charged. "I told him to stay one length away, at the most, off that horse - the only one he had to beat. And then he let him get away with 24 and 47. You can't do that!"

Cordero figured he might get away with such a tactic.

"Affirmed has been enjoying a slow pace all season," the Puerto Rican jockey observed. "That's the way Cauthen's been riding him every race. So I thought we'd make him run faster early, and see how he liked it."

Cordero kept Seattle Slew well out from the rail, turning for home. "This was a championship race. I didn't want to take any chances. I wanted to leave no doubt about which was the better horse," the rider said.

He did. Seattle Slew paid $6.20 straight as the second choice and, as the defending national champion, reintroduced himself into the Horse of the Year race. There is reason to believe, however, that the Marlboro marked the last appearance in competiton by the dark bay son of Bold Reasoning.

"Next time, we'll get him," said Lou Wolfson, the owner of Affirmed. He was referring to the weight-for-age Woodward Stakes here Sept. 30. But Taylor indicated Slew won't be in the Woodward lineup.