He is still asked "Why?" and Sen. James G. Abourezk, Democrat of South Dakota, has plenty of answers for those stunned by someone who would leave the Senate by choice.

He was he says, tired of "marginal victories - if you win at all -", he was tired of putting politics over family, he was depressed at the trimming and compromising necessary to get elected, stay elected and operate as a member of that Exclusive Club of 100.

On a recent Friday afternoon - the Senate business all but halted as his colleagues scrambled back home to woo constituents - Abourezk sat with his feet up, near the guitar case in his office, polluting the air with fat cigars fresh from a recent Cuba trip. "One of the joys of not running" was no longer having to make that weekend exodus to the constituents, said Abourezk, who announced this year he would not seek a second six-year term.

Abourezk discussed the "indispensability myth" that he feels governs so many politicians.

"I started out thinking that if I didn't do it, something wouldn't get done. I was rationalizing so that I could put my family out of the picture - and still not feet guilty about it. Nobody is indispensable - but once a politician makes up his mind that politics comes first, he'll use that so called indispensability as a rationale to neglect everything else."

There are skeptics in Washington who question Abourezk's motives. Friends and enemies in South Dakota feel he would have a very tough time in his next race, but generally feel he'd win if he hustled hard enough. The son of a Labanese peddler, Abourezk has been a lone voice for the Arab position in the Middle East, making him a certified hero to the estimated 2 million Arabs in the United States and to leaders of Arab countries. The conventional guess about Abourezk's future is that he will become a lobbyist and get rich off the Arabs.

He shrugs and jokes, "I hope I get rich off the Arabs - because it sure as hell isn't going to be the Jews."

He plans to open a law office hoth here and in South Dakota, may act as a political consultant, but never a lobbyist, he asserts. He says angrily, "The Israelis have been telling everyone that is what I'm going to do. I'm not going to be a lobbyist for anybody. It's a . . . job, to begin with. I don't think I have to get into that kind of living."

The Club on the Hill has long taken its lumps - from constituents: from an outside politician who last year made establishment politics such a successful target that it helped get him into the White House. Now the attacks come from within - from A (Abourezk) to Z (Zorinsky.) Sen. Edward Zorinsky, a freshman conservative Democrat from Nebraska, was here only a few weeks when he made front-page headlines. Zorinsky walked out of a closed-door session of the Senate Democratic caucus and told all. The hush-hush time-consuming subject was whether to charge for the haircuts senators now get for free in the Senate barbership. Zorinsky's frustration with everything from haircuts and vote trading to what he feels is creaking inaction almost caused him to quit after six weeks.

The other day he continued his criticism. "There's a letter on my desk asking if I want the key to the deck on the Dirksen building. It's very hidden - there's a map with a real circuitous route to get there. Imagine what the constituents would think, with the energy crisis, unemployment and inflation, if I'd be sitting up on a sun deck somewhere." Zorinsky plans to "stick it out" for one term. "It beats being unemployed."

That's the kind of heresy the press loves and is seconded by a few colleagues. Strom Thurmond told Zorinsky it didn't take him long to find out how things worked on Capitol Hill. Abourezk told him "I thought of most of the things you said after only two months here myself and wish I'd had the guts to say so then." Since Abourezk announced his retirement he has been approached by several ambivalent "closet quitters," who said they felt the same as Abourezk but felt trapped by personal desires into running again.

When Abourezk, now 46, came to the Senate in 1973 he was considered a renegade from the West - a wildeyed populist liberal from a conservative state. He is 5 foot 9, laughs easily, and is a vegetarian, in part because of his frequent battles to lose weight. Off hours, he prefers to smoke cigars, drink beer and pick guitar with staff rather than sepnd time with colleagues. He delights in one-upping those who feel that pomposity is a just reward for office holding.

"Sen. Byrd hates 'staff' and thinks they should be banished from the floor," said one senator's aide, referring to the majority leader. "One day a friend of Jim's an aide, came on the floor and Jim saw one of Byrd's guys advancing to bounce him. Abourezk loudly called for the aide to come and talk with him - patting the empty chair of a senator next to him, thwarting Byrd's bouncer. That may be juts a silly story, but that's really how Jim feels about treating people up here. He's one of the few."

Abourezk feels his "selfish, self-centered" political ambitions caused heavy problems for his wife, Mary and their three children. He had been a representative for two years prior to running for the Senate. "Charlie was 17 then and he ran away from hom when he found out we were coming to Washington." But Charles was found, and he came with the family. None of them liked the move, said Abourezk.

"They lost a lot because I wasn't around," says Abourezk, who feels it is impossible to be a senator and not neglect one's family. He is pretty bitter" about articles in The Washington Post last year about Charles, now 24 and living on an Indian reservation, getting food stamps. Charles later said he had a 16-year-old orphaned Indian in his care. Abourezk said he deeply resented his personal life being 'brought into politics," that his son was an adult and not the politician. "I'd been having a struggle with Charlie about himself and his future. He was pretty rebellious, probably due to years of me not paying any attention to him. And then that story hit." Abourezk had made up his mind, and told friends four years ago, that he was going to quit. The publicity about Charles just added to his reasons.

Abourezk is asked whether quitting and his concern for his three children, who are now grown (Paul, the youngest, is 17 and at boarding school) isn't too little, too late. "It isn't just the kids, it's Mary. We want to live a more normal life. You know how it is for politicians' wives. They're left home alone so much. Thanking God she hasn't taken to drinking."

In a club increasingly populated with varying shades of gray corporate lawyers. Abourezk is downright vain about his colorful, drifter past. He grew up on an Indian reservation. "I probably was a racist when I was younger. I can remember saying things like 'we treat our Indians good here.' Good God!"

He ran a ranch and a farm, taught judo for a living in college, started a used car dealership that failed, sold groceries on the road, went to engineering school, dealt blackjack at the American Legion, owned a night club, was a surveyor, tended bar which, out in South Dakota, automatically meant you were a bounger. "Those cowboys would break beer bottles and threaten to stab you," he recalls. The phrase, "There goes Abourezk over the top," referred to no political victories. It was used by customers accustomed to Abourezk leaping over the circular bar to tackle drunken brawlers.

Abourezk delights in telling tales about the kind of clients he had as a lawyer, the kind Arnold & Porter never heard of. One, convicted of cattle rustling, took the back end off of a mobile home and loaded the cattle in, keeping the mobile home look, curtains and all. He was arrested, the story goes, when he got a flat tire and a helpful trooper stopped to give assistance. As the cattle rustler frantically said he didn't need any help, the cows started mooing behind the curtains. Abourezk later got the man off other charges and the man became so grateful that he told Abourezk's older brother, "I'm going to do your younger brother a favorsmeday I'm ging to hang a jury for him!"

Abourezk is a drifter, but he works hard. He is a tireless and effective campaigner, an aggressive, outspoken legislator who has' won the respect of his colleagues. He returns that respect and friendship and attacks the institution rather than individuals.

"Whoever you plug in up here, nothing ever really changes. It's the system itself. Roosevelt's axiom is repeated over and over up hero - you have to get elected before you can do anything' - and that view involves cutting corners, watering down views."

His philosophy is not well articulated but has earned him a reputation for championing the underdog. "I try to determine when the people are being screwed and try to help them." That's tough in the Senate, he says, where "the only way to get legislation passed is to do something that favors corporations."

His colleague, George McGovern, admits his own frustrations have been many. "Far and away the hardest thing I've ever done is come back to the Senate after running for the presidency. It's been an extremely difficult period.

"What is deeply troubling to me is Congress won't even act on a bill up here until the administration tells them what it wants.Phil Hart had decided to quit before he knew he was ill. He had despaired of getting through good legislation. Jim (Abourezk) is more impatient than the average senator. I guess the good ones who stay end up settling for partial victories."

McGovern is "distressed" by the new, younger members who McGovern feels may perpetuate the "get along go along" weaknesses of the Senate. "I see a cautious play-it-safe mentality. I may be a way of winning by finese rather than head-on confrontations, but it's also a treacherous 'gamesmanship' route."

Among the most cool is McGovern's former campaign manager, Sen. Gary Hart, Democrat, of Colorado. In 1973 Abourezk led a fight to take on the oil companies - it had been more than 30 years since anyone had pushed an oil divestiture bill in Congress. Abourezk got nowhere at first. But he lobbied his colleagues. Rising oil prices, dire energy warnings, reports of extraordinary profits of oil companies - were all to Abourezk's advantage. His new ally bacame Gary Hart, who took the cool approach while Abourezk took the approach." In 1975, Abourezk's amendment to break up the major oil companies almost passed, stunning both observers of the Senate and the big oil companies.

"Jim cast the issue in terms of evil oil companies, I cast it as a structural economic argument," said Hart. "When a person decides to quit, it is not as simplistic as whether he is a fellow who gets along in the club or not. It has to do with a personality type - the kind who is unhappy with struggling to drag 51 people along to get a bill through."

Abourezk's chief accomplishment, his friends like McGovern feel, is in "shaking things up. It's a useful role and we'll miss him. If he had a flaw it was spreading himself too thin."

Abourezk not only tackled big oil companies and pushed the Arab position, he championed human rights in dictatorships and fought hard for American Indians and farmers. He considers a major achievement the creation of the American Indian Policy Review Commission and is pushing for legislation recommended by his commission. It accuses the Bureau of Indian Affairs of abuses and paternalism, misspending money, calls for decentralization of BIA power and control by local Indian tribes of Indian affairs.

In his constant protest against Richard M. Nixon - frequently referred to profanely - Abourezk found himself the lone dissenter when the Senate confirmed Nixon's appointment of Elliot L. Richardson as Secretary of Defense, even though other senators had told him they would vote "no" too. McGovern joined him in voting against Henry A. Kissinger for Secretary of State. "I felt weird voting against Richardson; I like him. But that Kissinger vote is one vote I'm happy I made. he's a very bad guy. He's amoral."

Abourezk is now writing a novel about Senate life. He hopes to make enough money as a lawyer to build a home in the Black Hills. "I've never really been a part of Washington life, but to get clients you have to be known, and I'm known here."

He has to fight disgruntled friends who tell him he's selling out. "I understand, because I felt the same way when Harold Hughes left."

But Abourezk's answer today to those who say he is paving the way for a "pre-shrunk, Sanforized" politician is to remark, "Bob Dylan caught the same hell when he stopped writing protest music. His answer was his body is not a public trust."

Suggestions for changing the senatorial system are hard to come by. One is to limit the term of office. Abourezk says "that is not the way to break up the cycle. We should limit the amount of time you can serve as a committee chairman or a ranking member. Once you've lost all those perks," says Abourezk, his brown eyes gleaming mischievously, "you'd see a big turnover. We need to get rid of this concept that once you're here you have to stay here."

Most senators however, as McGovern says, "think it is a helluva job."

One senior staff member of a major committee adds, "Abourezk is one of the few not infatuated with the mere role." He says, onlu half joking, that "we need a constitutional amendment making a senator ineligible to run for the presidency. Then you'd attract a different kind.

"The sad fact is that, while Abourezk talks about no victories and no accomplishments, most up here don't have a larger purpose in life than getting elected," he continued. "They are delighted to be senators. That's 'accomplishment' enough."