The tight-lipped insistence by the men who run South Korea that a discredited dissident must die, no matter how much revulsion in the West it excites, is building toward a climax with tragic consequences for U.S. interests.

It is by no means ensured that the death sentence imposed on Kim Dae Jung will be commuted by President Chun Doo Hwan. To the contrary, presidential advisers say Chun must hold firm or this country will never achieve true stability; dissidents, they say, will perpetuate political turmoil by using Washington as a court of last appeal.

This advice fails to appreciate the consequence for South Korea of executing a former presidential candidate on sedition charges. While top officials here told us they can weather U.S. economic reprisals, they fail to realize that hysteria spawned by the execution could undercut the U.S. commitments to defend South Korea -- upsetting all security considerations in Northeast Asia.

The winner would then be North Korea's Stalinist dictator Kim II Sung and his campaign to isolate South Korea. After the execution of Kim Dae Jung, the siren song from Pyongyang -- that Kim's communist garrison state would not harm its southern brothers if only those 39,000 American troops got out -- might sound sweet in Washington.

It is ironic in the extreme that the 30-year-old U.S.-Korean alliance, forged on bloody battlefields and honed by political stress, could be imperiled by the fate of Kim Dae Jung. He may be viewed as a lion in a certain Washington and Tokyo circles. But in Seoul, even opposition figures write off Kim as an opportunist and incorrigible intriguer, whose reprehensible conduct following the assassination of President Park Chung Hee last October helped provoke the army takeover.

The deeper irony is that, save for the Kim Dae Jung case, prospects are good for the new regime to achieve a blend of stability and democracy. Western diplomats and opposition figures, while not happy about the regime's more repressive aspects, see Chun Doo Hwan as a therapeutic leader.

The 49-year-old career officer who seized power last December and took over as president this month has the potential to achieve what Park Chung Hee lacked in 18 years of rule: genuinely popular leadership.

While Westerners scoff at the military regime's purification program that has cracked down against both corrupt politicians and racy television programs, it is popular with Koreans. They are delighted that public officials no longer spend Sunday at the golf course followed by an orgiastic 19th hole. In place of politicians, the new cabinet is composed largely of well-regarded technocrats.

Oh Se Eung, one of the most outspoken opposition members in the National Assembly during the Park regime, was sickened by Kim Dae Jung's conspiratorial conduct during the past year. Oh would have preferred true democracy, but accepts President Chun's promise of order now and democracy ahead.

Similarly, Chun has a higher rating in the U.S. Embassy than he himself might imagine. U.S. Ambassador William Glysteen, like Congressman Oh, is a realist -- unlike missionaries and intellectuals who would impose American-style democracy on South Korea. But he has tried to impress on the regime that even conservative circles in Congress would be appalled by what they would consider a political execution.

Glysteen's advice so far goes unheeded, pointing up how little two very different cultures have learned about each other during 30 years. The Americans cannot understand how much Kim Dae Jung's defiance of Confucian order outrages the regime. The Koreans cannot understand how killing one shopworn politician could endanger this old and mutually vital alliance.

The Washington-Seoul collision course could be avoided if Kim's life is spared before the death sentence reaches Chun's desk. Kim could save himself by confessing error, but officials here believe he is counting on the West to reprieve him. There is a frail hope than an appeals court will save Kim from the gallows.

The affair is an extraneous farce that menaces what might be a final chance for South Korean stability. "I think we are playing the last card," an influential official confided to us. That play might well be doomed by killing a discredited politician, bringing changes in the Asian power balance of profoundly alarming dimensions.