ONE OF THE OLDEST saws about Southern politics runs something like this: If only a politician in the South could run with the united support of blacks and blue-collar whites, he would be unbeatable. And since he wouldn't be tied to the rich whites who control the South, he could really change things. Only a very few Southern politicians have been able to put together this mythical coalition, but Jim Garrison, the six-and-a-half foot tall New Orleans district attorney who lost his third reelection campaign in December, remained politically powerful in New Orleans for years with a loyal black and blue-collar white constituency.

But because his career has been so bizarre, it's not likely that anyone will ever herald Garrison as a champion of New South politics. Garrison became the district attorney in New Orleans in 1962, winning a surprise victory over an entrenched, conservative incumbent. He spent his first term cleaning up prostitution and gambling in the French Quarter and getting in fights with criminal court judges, and he was reelected to a second term in 1965 by a wide margin.

Some time in the first year of his second term--no one is sure exactly when--Garrison became convinced that Lee Harvey Oswald had not assassinated President Kennedy on his own. Garrison decided that Oswald was just the pawn of an elaborate conspiracy run by the Pentagon and the CIA, based largely in New Orleans.

In the middle of 1967 the New Orleans newspapers figured out that Garrison was spending almost all his time investigating the assassination, and he became a national figure overnight, the golden boy of the conspiracy buffs. A few months later, Garrison indicted Clay Shaw, a quiet, wealthy New Orleans businessman, in connection with the conspiracy, and once that happened his troubles began.

It was clear from the beginning that Shaw was innocent. Garrison's star witnesses were a heroin addict who claimed he saw Shaw and Oswald together once when he was shooting up at the lakefront; a businessman who remembered the details of conversations with Shaw after Garrison's staff hypnotized him; and an accountant who fingerprinted his children every morning to make sure the CIA hadn't stolen them during the night and substituted lookalikes to spy on him. The national and local press gave the trial heavy coverage. Garrison lost and came out looking like a fool.

A few months later, Garrison ran for reelection again and won by a landslide, beating a bland liberal named Harry Connick. The two New Orleans dailies had blasted Garrison almost daily on their editorial pages, and few major political groups had endorsed him. Garrison hardly campaigned at all; he spent almost all his money on three enormously effective 15-minute T.V. spots the night before the election. He was an awesome figure on T.V. He wore a black suit and sat in a black chair in a room with black walls, so that his face seemed to hover in mid-screen, and delivered his pitch in a deep, smoke-cured voice. At the end of each spot he would bring on his family and then dismiss his wife and daughters and fondly drape an arm around each of his two sons.

"Boys," Garrison would say, "do you want to grow up to be president?"

The boys would fudge a while and Garrison would say, see, American kids can't dream of being president any more because they're afraid they'll be assassinated. The point was that Garrison was working toward an America where kids wouldn't have that fear.

IN HIS THIRD term, Garrison did little work on the conspiracy investigation and was in fact out of the limelight for a couple of years--until last summer, when he went on trial for bribery. The federal attorneys seemed to have an open-and-shut case against Garrison, because they had tape recordings of him accepting bribes. But they had underestimated their quarry. Midway through the trial, Garrison--who would sit in court all day reading The Best and the Brightest and underlining his favorite passages--fired his lawyers and took over his own defense. He completely bowled over the jury, and was acquitted.

Meanwhile, Harry Connick had been biding his time, waiting for another shot at Garrison. He ran again against Garrison last fall, and won by about 2000 votes with a big law-and-order campaign. Garrison spent most of December and January challenging the election results in court, but finally gave up the ghost three weeks ago, his career in New Orleans politics apparently over.

Garrison didn't get many affectionate postmortems. By the time he lost the Shaw case, he was a genuine pariah, the only major political figure in New Orleans that the establishment press felt safe attacking. Garrison eventually became something of a safety valve for other politicians--the media would spill all their venom on him and by and large leave other politicians alone. In the same way, nobody really questioned Harry Connick's fervid law-and-order stance, reasoning that as long as he was against Garrison he deserved unified support.

This is not to say that Garrison is a good man. For at least his last six years in office, he was a negligent district attorney, and his indictment of Clay Shaw was totally unjustified. But his unique bi-racial constituency, though helpful in winning elections, lost Garrison the backing of the daily press and major financial institutions. Because of this he ended up absorbing some of the criticism other politicians should have gotten. The main drawback, in fact, of the black-blue-collar-white dream coalition is that it will always face a hostile establishment. The only other politician in New Orleans whose constituency roughly approaches Garrison's, a councilman named Eddie Sapir, is another favorite target of the daily New Orleans press.

Garrison never stated very clearly the conspiracy theory he spent so much time investigating. After he lost the Shaw case, he wrote a book, A Heritage of Stone, about the Warren Report and the CIA's involvement in the assassination, but a cohesive theory never emerges--probably because Garrison wanted to avoid libel suits and couldn't get access to the secret files he needed to prove his case. The book ends up saying that the CIA and the Pentagon wanted Kennedy out because he was trying to bring peace to Southeast Asia. They formed a conspiracy--Garrison hinted that Lyndon Johnson was part of it--to kill Kennedy so they could go on escalating the war and building up a military state.

A Heritage of Stone is full of statements that sound very attractive right now; "Why was the government lying to the people?"; "No one wants to recognize that somewhere along the line America has ceased to be the home of the brave and the land of the free."; "When a powerful government takes a stand against the truth, other elements of the power structure may join in the defense against the common enemy." The real theme that emerges among all the conspiracy theories is a deep mistrust of strong, centralized federal government.

In that sense Garrison is the descendent of a long line of Southern political feeling. Despite his sober black suits, his erudition and his conspiracy mania, he is appealing, like generations of Southern politicians before him, to Southerners's fear of being controlled by a hostile, unsympathetic, and still foreign Northern nation.