IF YOU BUY what Edwin Diamond and Bruce Mazlish are selling in this political year bio of Jimmy Carter, the president proved on Tuesday that he is both a part of, and apart from, the people of New Hampshire. Carter is running for reelection, the authors would tell us, because of his deep psychological need to "measure up and win." The man from Georgia picked up his 49 per cent by carefully managing to control his emotions and quell the steamy contradictions which threaten at any moment to split his newly-parted scalp. Carter got to the White House through his belief in God and, a la Norman Vincent Peale, by the power of positive thinking.

This is a terrifying book, one that everybody who plans to vote for Jimmy Carter should be force-fed before he pulls the lever. The authors' conclusions strain the obvious ("The reality is that both parents 'shape' Jimmy Carter, just as most of us are influenced by both mother and father"), but what lies between the lines is genuinely scary. "We believe that Carter really believes in his own promises, and his desire to bring compassion and justice to the victims of the sinful world," Mazlish and Diamond conclude; it's just that the angels watching over the Oval Office can't get it all together.

ERRANT IN their path, blinded by the realization that Jimmy Carter is God's personal representative on earth, the authors indulge in baths of high-school psychology. From his childhood in Georgia, little Jimmy carries the entire burden of Southern history to Annapolis, where he grits his teeth for three years. Laboring under the shadow of his father, Carter develops the win-or-die attitude of the killer politician. Crushed by his failure to win the governor's seat the first time around, Carter goes off to find himself, "a star pupil in the self-improvement school."

Carter trusts the people because, finally, he trusts himself. He and the people merge into one. This revelation came to him during the process of the born-again experience, when he went door to door. Since he healed divisions in his own psyche, he feels able to be a healer/leader to his countrymen.

Who is Jimmy Carter? "He is a complex, contradictory personality," Mazlish and Diamond say. They continue in a flash of insight: "Most of us, of course, are complex and full of contradictions." Carter's brand of neo-populist rhetoric and waffling reflects those contradictions. Expediency exists as part of Carter's "realism." The president blurs the lines between liberalism and conservatism because he must be "true to his own character, with its basic need to embrace contradictions." Carter's remembrances of his downtrodden childhood, his career as a "nuclear engineer," his faith in Bert Lance--all explained.

He seems compelled to project his inner feelings--his emotional reality--outward as a description of events and external reality. He does not 'lie'; but he shapes the world according to a 'deeper' truth, his own needs.

Or later: "He chooses to suppress--the sin of omission. He does not lie to us." The authors dismiss the other interpretation--that Carter is just another hack politician in the Bilboesque tradition--in a series of short clauses and brief references.

At times, Mazlish and Diamond descend from the simplistic into the banal. In various parts of the book, the authors compare Carter to DeGaulle, Gandhi, the young Luther (a la Erik Erikson) Oliver Cromwell and Handsome Lake, a Seneca Indian who had a series of revelations that led to his belief in his own leadership. Where other presidents are heroes or policy-makers, Carter belongs in the ranks of "revitalizers." Bear with the authors' dangling prose:

The revitalization leader brings both political and religious sustenance to those in need; he converts his own profound religious experience to political purposes. In his mission of bringing spiritual rebirth, such a leader necessarily deals in symbols; he often derives his own power from the ability to communicate with supernatural agents. He talks directly with God often through prayer. An 'outsider,' he stands apart from the existing authority structure, and against the Establishment, deriving his powers from his experience with the supernatural.

BUBBLING INSIDE those Baptist-cold blue eyes, however, is a heritage of whippings, humiliation, defeat, depression and finally--drumroll--rebirth. Since Mazlish and Diamond finished their book, however, Carter has strayed from the authors' picture of him as a model of control. "Carter's incredible competitive streak nowhere appears in any of the accounts of his White House days," the authors write. This about the man who said he would whip his opponent's ass. And "crisis thinking," the authors tell us, "runs counter to Carter's instincts." A terrific politician, perhaps--but a dismal leader.

Try as they do to protect Carter's image, the authors inevitably slip into the bottom line on the president. They label the chapter on his childhood "Just an ordinary little boy," and scattered throughout the remainder of the book are reminders of Carter's aggressive normalcy. "Carter fits in the American grain of thousands of upwardly mobile men and women in business and sales." Or, as Diamond concludes, "Jimmy Carter is like thousands of other middle-aged men of middling stature in authority in our society. He is the man next door--if you live in the nicest neighborhood in a nice small town or a 'good' suburb." Only iron gates and political smarts separate Carter from 100,000 other people. And with the president ahead in the polls, this is nothing but depressing news.