RICHARD NIXON is said to have pronounced that, of all the president's men, only Alexander Haig and John Connally were qualified to be president. Watergate Special Prosecutor Leon Jaworski said that for a while Haig actually was "Our 37 1/2th president." Haig does not conceal his ambition to be president. He ran, or at least limped, for president in 1980. No doubt he would run again if he thought he was a credible candidate.

Indeed one of the coherent explanations for his resignation is that he could see himself being treated by the Californians in the White House as he and Henry Kissinger treated William P. Rogers, to the point where Haig's presidential prospects wld fade away.

This is, however, no campaign biography. It belongs rather to that new and select category of political literature, the anti-campaign biography. In this genre, instead of the candidate smiling his way from log cabin to White House, he snarls from early psychohistory through one intrigue after another.

Roger Morris watched Colonel Haig, as he then was, from close up when they were colleagues on the National Security Council staff in 1969-70. Morris, a Harvard Ph.D., was a holdover from the previous administration; he resigned, along with others, over Cambodia in 1970.

It is apparent that he did not much like what he saw of Colonel Haig in those days; and distance, and diligent research, have not lent enchantment. Morris uses as an epigraph a line from Henry James' The American: " 'I may be dangerous,' he said, 'but I am not wicked.' " It is my clear impression that Morris is more sure that the first half of that sentence applies to his subject than that the second does.

The biographical background and Haig's early life behind lace curtains on the Philadelphia Main Line are sketched in surely and with some sympathy. Haig, in psychohistorical terms, came from a background that might have been designed to produce a hard, angry careerist. The parallels with the early life of Richard Nixon are striking. No doubt similar origins have also produced saints and sunny Jims.

Haig's father was a rising lawyer in Philadelphia who died when the son was an adolescent. The mother was Irish, ambitious and, after her husband's death, if not poor, at least in what used to be called "straitened circumstances." Through political pull, Regina Murphy Haig got the boy into West Point. A brother became a priest. Morris has an intriguing theory that the general's malapropistic misadventures with words result from the fact that during World War II West Point cut right back on such inessentials as English.

At any rate, Haig was too young to fight Germans or Japanese. His first experience of the military profession was as an aide-de-camp at the court of General Douglas MacArthur in Tokyo. When he called his mother on the Main Line and shouted over the trans-Pacific cable, "I'm going to get married!" She shouted back, "Is she Japanese?" Mrs. Haig need not have worried. Her son married a general's daughter.

Morris interprets Haig's style in terms of the prevailing "careerism" of the post- 1945 Army. It was no doubt a time of anti-climax, even of cynicism. Haig's next general, whom he served in Korea, had lost a son in World War II and used to ask his young aides bitterly, "Why are you alive and he is dead?"

The glorious victory had been won. Judged by that standard, nothing could ever be so good again. All that was left, except in the grim campaigns in Korea and Vietnam, was a sometimes sordid hunt for patronage and promotion.

Morris may be unjust in characterizing the U.S. Army as essentially carreerist. Yet, Haig certainly was. When he arrived in the West Wing of the White House in 1969, a Pentagon colonel with good connections, the civilians who stood in his way were simply no match for his bureaucratic wiliness, hardiness and sheer determination to survive.

And survive he did. The most controversial, and the most original, contribution of Morris' book is the case he deploys for believing that Haig was deeply implicated in the secret inner history of the Nixon White House. It was not just a matter of clandestine foreign policy. Haig, Morris argues, was privy to the "Track II" plotting that led to the assassination of Chile's army commander General Rene Schneider in 1970. Haig was the sole liaison between the White House and the Joint Chiefs during the secret bombing in Cambodia and was the principal advocate of massive bombing in North Vietnam, and of a punitive strike on North Korea after a Navy intelligence plane was brought down over the Sea of Japan.

But he was also involved in the internal intrigues of the White House. The myth is still prevalent that there was an impenetrable wall of separation between the "White House horrors" on the domestic front, and the foreign policy operation run by the president, Kissinger and Haig, which was in contrast noble, patriotic and above suspicion. It is a myth both Haig and Kissinger have used to advantage. Morris does not buy it, and he has got new material, both on the question of responsibility for the wiretaps, and on the larger issue of responsibility for the atmosphere of illegality that was the essence of Watergate.

If you are an admirer of the general, you will read Morris' account with indignation. But his broad point is compelling. It was the secrecy on which the Nixon- Kissinger foreign policy critically depended that bred obsession with leaks, and created the plumbers. It is therefore implausible that Haig, the man charged with responsibility for security on the National Security Council staff, should not have been privy to the White House's counter-espionage efforts, and incredible that he should have escaped untainted by his involvement.

Morris also argues convincingly that it was Haig who cut the deal that led to the Nixon pardon, and so to his own survival and future career as NATO commander, presidential candidate and secretary of state.

Ten years later, it is the fashion to ask of the Watergate culprits, where are they now? Haig is unscathed. Seven years after the break-in, someone did have the gall to ask him about the affair. "It is sort of ludicrous," was the general's reply, "that a decade later questions are still being asked." 'Twas in another country, and besides the wench is dead.

Roger Morris has performed a service in reminding us that a public man is the sum of all he has done. The fallen secretary, the perhaps-future presidental candidate, is still the assiduous staff officer with a talent for intrigue and no great squeamishness about the dirty jobs. "He was always perfectly comfortable doing what must be done," a colleague said.

The general's progress so far casts doubt on the idea that his resignation was motivated by high principle alone.