SEOUL -- Euphoria over South Korea's abrupt switch from authoritarian rule to democracy has given way to apprehension that next month's presidential election results will not be accepted by the losers.

The opposition plants seeds of chaos by saying in advance that the government candidate, Gen. Roh Tae Woo, can win only if the election is rigged (though he may well finish first against divided opposition). Restrained by the 1988 Olympics scheduled here and massive U.S. disapproval, the military is unlikely to prevent rival opposition candidates Kim Dae Jung and Kim Young Sam from taking office if either finishes first. But bitterness toward Kim Dae Jung by an officer corps accustomed to power is not promising for Korean democracy.

While South Korea turns away from authoritarian rule, the presidential candidates reenact politically the long, bloody struggle between generals and dissidents. After achieving an economic miracle under 27 years of military rule while confronting pressure from hard-line communist North Korea, this country is uneasy facing the dangers of democracy.

Uncertainty has replaced predictability. Nobody takes seriously government polls showing its candidate ahead and Kim Dae Jung a poor third. His own supporters confess Roh has not successfully distanced himself from the unpopular presidency of Gen. Chun Doo Hwan, whose succession of political blunders built a crisis that inadvertently brought the turn to democracy.

The last blunder was the president's anointment of his right-hand man, without even a token party election. Roh has not countered the opposition's slogan, embodied in four large Chinese characters: ''End Military Dictatorship.''

We found Roh working hard at ''distancing'' in his first Seoul rally, claiming he can best keep the military out of politics. As we approached the stadium just before Roh began speaking at 2:30 p.m., thousands who had arrived as requested at 11 a.m. were bleeding away. The 40,000-seat stadium, not filled when he began, was emptying fast before the candidate hurriedly finished his speech.

On the next day, we traveled to Pusan and saw upwards of 200,000 wait patiently for hours in a muddy field under threatening skies to hear Kim Dae Jung. The intensity of both the speaker and the audience was markedly greater than at the Roh rally.

Roh is in trouble if either of the Kims drops out. One key opposition leader calls that a 50-50 chance, and another flatly predicted to us it would happen about 10 days before the election if it seems Roh will win. But how will they really know? And which Kim will make the sacrifice?

The difficulty of answering such questions accounts for Kim Dae Jung's advance preparations to call foul. One adviser, Dr. Yoo Jong Keun, told us he sees a duplication of ''the Philippine disaster'' when ''people power'' overturned a bogus vote count. That confirms the instinctive assessment of Oh Se Eung, the senior national assemblyman and an independent member of the ruling party: the opposition is trying to replicate Corazon Aquino's rise to power.

Even some opposition politicians concede no real likelihood of vote fraud. Roh's campaign tries to avoid provocation. Walking back from Roh's Seoul rally, government supporters demonstrated against ''End Military Dictatorship'' posters at a women's university. But the instant a thrown rock broke a dorm window, riot police appeared from nowhere to protect anti-Roh students.

Nor is there an antigovernment nucleus in the officer corps here that was essential to Aquino's takeover. The danger is that protracted demonstrations against an apparent Roh victory could trigger a hard hand by the military to ensure order.

The defense minister, Gen. Chung Ho Yong, told us postelection turbulence would be a ''a good invitation card'' to the communist north. That is a significant warning, since Chung, unlike many fellow officers, is a sincere democrat credited with talking President Chun into the abrupt turn toward democracy. He has ordered the armed forces not to interfere with the election outcome.

Still, to salute as commander in chief a man who has spent much of his life in prisons and under house arrest is a bitter pill. The officers loathe Kim Dae Jung, considering him procommunist, though they are never able to prove it. He is burdened by old pleas, made while in exile, to remove U.S. troops as a sanction against the military regime.

Actually, all the presidential candidates are blemished by the past. But Koreans yearning for a new generation of unmarked leaders first must get by next month's election without setting off a chain of events calamitous for Korea and the United States alike.