The odds have swung now to no better than even that Chun Doo Hwan, South Korea's faltering strongman, will achieve the tidy finish to his reign that he so badly wants. The massive public anger with Chun that has boiled over into violent demonstrations seriously imperils his plan to hand over power to a malleable successor next spring.

The direct threat to Chun comes not from the streets but from his fellow generals. Already disenchanted with him because of his increasingly arbitrary rule and an odor of corruption that wafts around his entourage, they will not blink at dislodging Chun if he stumbles in dealing with the public challenge to his authority.

No one knows this better than Chun himself. In an extended conversation early last year, he acknowledged that his desire to arrange a quiet and decorous retreat from the position of power that he seized in 1980 is driven by a fear of winding up like his presidential predecessors. (All of them have been thrown out of office or assassinated.) But Chun's insistence on staging his retreat on his own terms now endangers the entire enterprise.

Even if the demonstrations subside, Chun's power base has been dramatically eroded. Senior U.S. officials who hope Chun can survive into 1988 have concluded that he now is unlikely to be able to impose his crony, Roh Tae Woo, as his successor. They foresee instead an interim regime, declared by the generals or negotiated by them with Chun and the opposition, that would take power to draw up a new constitution.

This conclusion has forced a swift reappraisal by the Reagan administration of its once undiluted support for Chun. When the Korean leader provoked the crisis in April by breaking off the political dialogue he had established with the opposition, the administration issued the mildest of public reservations about Chun's actions. Distracted by the Iran-contra problems and later by the Persian Gulf, the White House was not up to focusing on a crisis that was only incipient.

But the demonstrations and the uncertainty created by Roh's eclipse have brought home to senior foreign policy makers that they are looking at an unraveling in Seoul that could present the Reagan administration as severe a challenge as it will face in its final months.

For, as it did last year in the Philippines, the administration confronts the dilemma of having to press a friendly regime to undertake reforms that are likely to end by bringing about that regime's demise. Washington's willingness to do this is an accurate measure of how desperate the situation in Seoul has become and how disastrous for the United States any other course would be.

By dispatching Gaston Sigur, the head of the State Department's East Asian bureau, to Seoul to call publicly on Chun not to declare martial law, the Reagan administration is reluctantly but knowingly depriving Chun of much of his leverage in trying to force the demonstrators off the street without using the iron fist.

While it is doubtful that anyone has walked into the Oval Office to explain it this way, the Reagan administration is in fact making human rights and the determination to avoid massive bloodshed the centerpiece of its efforts to maintain political stability and protect long-term American interests in a vital allied nation.

That is the way it was supposed to work in the Carter administration, of course, but never did. Compare Sigur's warnings about martial law to the Carter administration's public welcoming of the shah of Iran's turn to military rule in November 1978 in similar circumstances. More to the point, the Reagan administration is now actively attempting to head off a repetition of the kind of massacre of demonstrators by Korean troops that occurred in Kwangju in May 1980 -- on Jimmy Carter's watch.

Not plagued by arguments between policy makers about human rights vs. toughness, as was the Carter policy team, the Reagan administration has its priorities right in South Korea. Because of the deep American involvement in the operational control of South Korea's military, another Kwangju now could poison Korean-American relations for decades to come.

That would be tragic strategically and historically. The American record in helping prevent a takeover of South Korea by the hostile and paranoid regime to the north is one of our nation's finest accomplishments abroad in the past four decades. It should not be endangered now to enable Chun to try to follow his obviously flawed vision of the future.