SIX AND A HALF years after he resigned the vice presidency under a heavy cloud of criminal accusations, Spiro T. Agnew claims to have left largely because he feared he might be murdered. What he took to be a veiled threat of assassination, relayed to him through Gen. Alexander Haig, former President Nixon's chief aide, is the centerpiece of Agnew's forthcoming book, "Go Quietly . . . or Else."

[when informed of Agnew's suggestion, Haig, now president of United Technologies Corp. and recuperating from heart surgery, "laughed, said it was the most preposterous thing he had ever heard of, and would not dignify it by discussing it any further," according to a United Technologies spokesman.]

Agnew's home now is in Rancho Mirage, a haven for the rich and the near-rich not far from Palm Springs, Calif. His neighbors include Frank Sinatra, a good friend, and former President Gerald Ford, who succeeded Agnew as vice president. The Agnew condominium sits on the border of a golf course, and sometimes he sees Ford drive by in a golf cart. They do not speak.

On his way abroad, Agnew paused in New York last week to talk about his book. In a suite in the Waldorf Towers, he sat toying with a gold lead pencil, a reserved man with small, spare gestures -- in contrast to the drama of his prose.

The book title is not a direct quote. It is the inference Agnew says he drew on Oct. 4, 1973, after his military aide, Gen. Mike Dunn, came back from a meeting with Haig to report that he had been told that Nixon, besieged by Watergate investigators, could no longer support his vice president's fight to clear himself.

He quotes Dunn's memo of the session, reporting a Haig statement that, innocent or not, Agnew would certainly be convicted at a trial. If he allowed himself to be indicated, Haig is supposed to have said, "we are off to the races and cannot control the situation any longer -- anything may be in the offing. It can and will get nasty and dirty." There was also a reminder: "The president has a lot of power -- don't forget that."

Agnew writes that he took this as an "open-ended threat." He was scared. He knew from National Security Council meetings how the intelligence forces operated. "I feared for my life," he writes. "If a decision had been made to eliminate me -- through an automobile accident, a fake suicide or whatever, the order would not have been traced back to the White House any more than get-Castro orders were ever traced to their source."

When it is pointed out that Haig may have been thinking of prison rather than assassination, Agnew's face hardens, and he hedges. He says that he did not write that Haig actually had threatened his life, only that Haig's remarks had "worried" him.

"You can imagine how, after months of this pressure cooker" he says, "this kind of statement relayed coldly and dispassionately was a worry to me. I didn't know what it meant. I thought it was entirely possible that I could be in danger, based on my frame of mind at the time."

Actually, Agnew's book does not describe his military aide as cold and dispassionate. It says Dunn was "tense and upset." Asked how Dunn interpreted Haig's warning, Agnew brushes away the question with, "You'll have to ask Dunn about that."

Dunn, now retired from the military and president of the Can Manufacturers Institute, says, "In my mind, there was never any threat of bodily harm. The idea never entered my consciousness."

[As best he can remember, Dunn says, the wording cited from his October 1973 memo is essentially correct. But as he recalls, the statement that "The president has a lot of power -- don't forget that" was made in a very different context. "The context there was that if Agnew played ball and went quietly, maybe things could be worked out, that he wouldn't have to go to jail."]

["In all fairness," Dunn says, "the man was distraught at the time, as he had every reason to be." But Dunn cannot resist chuckling at the notion of an auto-accident or fake-sucide assassination attempt.]

["I read spy novels as much as everybody else does," he says, "but I wish our intelligence agencies were one-third as effective as they are in them." Besides, he adds, "Agnew was under Secret Service protection at the time, and our intelligence services are rivals. . . The Secret Service prides itself on not losing clients. . . Can you imagine the kind of imbroglio we would have had if one branch was going after another?"]

The former vice president is convinced that Haig knew Nixon eventually would have to go and did not want Agnew in the line of succession. Agnew never elaorates on this view in the book and doesn't seem to be able to explain it in person. The book bristles with references to "my enemies" and to left-wing forces bent on destroying Agnew once he had achieved eminence as a voice of conservatism, a scourge of war resisters and liberal journalists. But nobody ever accused Haig of liberal leanings.

Agnew confronts the issue with a hint of impatience and some opacity. He doesn't know what Haig's politics are. He isn't even sure for whom Haig spoke. "Haig may have had other affiliatons," Agnew said quietly. "How do I know who he preferred in there? As a matter of fact, I don't know if this was Haig acting for Nixon or Haig acting for Haig."

In the time of his troubles, Agnew entertained some ambivalence about his president. On the one hand, he admired Nixon's politics, except for his China policy and the SALT negotiations. On the other hand, it dismayed him that pleasant and supportive encounters with Nixon would be followed by visits from people like Haig and Bryce Harlow, another White House adviser, to discuss Nixon's urgent need for Agnew's resignation.

Only later, Agnew says, did he realize that, at the outset, he had antagonized the president by offering dissenting opinions at Cabinet and NSC meetings. He ceased after H.R. Haldeman, Nixon's first chief of staff, told him that Nixon did not care for his contributions.

His conflicts about Nixon abruptly disappeared with his departure from office. Agnew was especially offended by "the celebration" of the choice of Ford to succeed him. "I felt it was rather callous, particularly since, for over a year, I never heard from Nixon to say, 'How are you? What are you doing?' It was as though, from the moment I stepped out of office, I had ceased to exist."

In the fall of 1974, a few months after Nixon resigned, Agnew was in Palm Springs visiting Sinatra when a call came from the former president's San Clemente home. "I didn't take the call," Agnew says. "I just thought it was a little late."

He is not a forgiving man? Agnew hesitates and then, his face set, says, "No, I guess not."