I'VE ALWAYS HAD a hard time deciding whether Martha Mitchell was a spunky, savvy lady who divined the truth about Richard Nixon earlier than a lot of other people or simply an impossible, unreliable, self-destructive pain in the kazoo. Mostly the latter is the word from here after reading her biography by Winzola McLendon - maybe because McLendon has devoted 200 pages to the gruesome details of Martha's decline and fall.

Not that Martha Mitchell didn't have her good moments and her engaging qualities - at least in the beginning.She was good-hearted and lively and outspoken. But these are rather insubstantial (as mediocre, in their way, as her little vices: fibbing, eavesdropping and lusting for publicity), and under pressure they gave way to her overwhelming feelings of insecurity. When the going got tough, Martha Mitchell simply fell to pieces.

At the end of her life when she was sick, broke, deserted by her husband and drunk almost as often as she was sober, Martha Mitchell said, "I never would have been in a predicament like I am if I hadn't . . . left the South." Don't you believe it. If anything is clear from this version of her life, it is that Martha's craving for center stage and her emotional fragility made her ripe for a "predicament" no matter where she lived. We would simply have been spared the details had she not gone north and tangled with John Mitchell and Richard Nixon.

The details are what we get from McLendon, a Washington newspaperwoman who logged dozens of hours talking to Martha Mitchell as a reporter, literary collaborator and long-suffering friend. She tells the story of Martha's rise to become "the most talked about, written about, adored or hated, praised or maligned, admired or ridiculed Cabinet wife in the history of the United States" with thoroughness and considerable sympathy. Martha's crack-up is described with an explicitness that will make you wince.

Martha Mitchell became an instant celebrity in 1970 when, as wife of the attorney general, she took to the airways and denounced anti-war demonstrators as "very liberal Communists" (remember them?). The rest is history and it's all here: the late night telephone calls, the outrageous sayings, her rise in the public opinion polls until she became the most popular speaker in the Republican Party next to Richard Nixon. Then came Watergate. With her inside information (eavesdropping on John's conversations gave her the first word about "Dirty Tricks") and her penchant for talking to the press, Martha became a hazard. She was stonewalled - imprisoned in a California villa and given the famous "shot . . . in the behind." It was the one thing she never forgave John Mitchell.

After Watergate, it was downhill all the way for this star-crossed pair. As the cover-up unravelled, so did they. Their final months together had some colorful moments. One evening late in 1972, Winzola McLendon got a frantic call for help from Martha. She rushed to the Mitchells' Watergate apartment to find John looking "like he'd been in a brawl," and Martha claiming that he'd roughed her up. She had "large red marks on her arms" and the cap was knocked off one of her front teeth - the very teeth, she said, "he made me ha ve . . . capped." In 1973 when John walked out on Martha (he left without saying goodbye and she never saw him again), she took his portrait from the wall and "with turpentine and such kitchen supplies as SOS pads, Ajax, Clorox, mayonnaise and Heinz catsup, Martha erased John Mitchell's face from the canvas." Later, she asked a friend to get rid of it and he obliged by dumping it in a trash can down the street.

John Mitchell will never get a Hubby of the Year Award for his performance in this biography. According to McLendon, the first indulged his wife, then exploited, later ignored and finally abandoned her. When he was sentenced to prison for his part in the Watergate cover-up, he said, "It could have been a hell of a lot worse. They could have sentenced me to spend the rest of my life with Martha Mitchell." Good old John. But then, neither was Martha's behavior exactly inspirational. It turned into a round of all-night rampages, blackouts, and rages - laced with drugs and drink. During a single day, flying from Atlanta to New York, Winzola McLendon saw her put away five single and four double martinis. By the end, she was alienated from her husband, her daughter and intermittently from her son. It was left to a small group of friends - Winzola McLendon among them - to pick up the pieces. They took turns sitting with her, talking her out of suicide, looking after her when she became ill with multiple myeloma. In her last week she was so penniless that they cashed the little checks sent to her in letters from sympathetic strangers (13 of them totaled $158) to pay for her final load of groceries. She died alone in a New York hospital in the spring of 1975.

Martha Mtichell's story is a pitiful tale. Nothing, except morbid curiosity, would draw one to it. She was only a bystander (albeit a noisy and dissenting one) at a national disaster in which she played no major role. "I have never done anything wrong in my life, ever," she protested to one of the reporters camped outside her apartment after John pleaded not guilty in the Vesco case, "I wasn't even sent to the principal's office when I was in school." Her life is only occasionally interesting, never edifying and simply not substantial enough to warrant our attention for 400 pages of a full-length biography.

For all that, I must admit I miss her. She brought fun to a bleak administration and to a starchy city. Take the story, told here, of the early Nixon years when members of the cabinet had a secret dinner meeting with a very important foreign official: "The men were in the drawing room, having the meeting after dinner, and the wives kept out of the way in another room. But Martha wandered off somewhere and came back downstairs wearing a hula skirt. She danced into the room where the men were meeting and wouldn't leave. John Mitchell didn't say a word. . . ."

We could use that in this town - at least three nights a week. And maybe more, now that Billy Carter has cleaned up his act. CAPTION: Picture, Martha Mitchell, AP