Disunity dogs Korea opposition. Despite turmoil, constitutional amendments pave way for election by end of year. SOUTH KOREA ELECTIONS

South Korean politicians can agree on only one thing these days - that there will be direct presidential elections. On Monday, in a rare moment of unanimity, the National Assembly passed the constitutional amendments that will make possible the December vote. Aside from that, however, disunity and dissonance are the dominant themes of Korea's fragile experiment with democratic political change.

The electoral process enters its final phase with the opposition party apparently hopelessly split, fielding two candidates.

This past weekend Reunification Democratic Party president Kim Young Sam officially declared his candidacy. His rival, Kim Dae Jung, has not formally followed suit but has made clear his intention to run. Meanwhile, the ruling Democratic Justice Party (DJP) standard-bearer, Roh Tae Woo, faces a challenge from a fourth candidate on the right. The four-way race, Korean political analysts fear, will not produce a clear victor with the support of a majority of the electorate.

The legitimacy of the minority government would certainly be challenged by the losers, creating the prospect of instability in the aftermath of the election.

The focus of concern is the division between the ``two Kims,'' as the leaders of the antigovernment opposition are oftencalled. After the tumultuous events of this past summer, when massive street demonstrations forced the military-backed regime to concede to demands for democratic reform, there was hope that their longtime rivalry would take a back seat to the goal of toppling the regime. Both men publicly pledged there would be a single candidate, though neither yielded as to whom that might be.

For now, their rivalry is unchecked. On Saturday Kim Young Sam will attempt to demonstrate his ability to draw massive crowds - his rival's forte. He will hold a rally in his hometown, the southeast industrial city of Pusan, where he predicts a turnout of 1.5 million. Kim Dae Jung plans to take a tour that will bring him to Pusan the day before.

At this point, says one Western diplomat, ``It's quite clear that the ambitions of both sides make it impossible'' to agree. Both men declare that they alone best represent the ``people's desire,'' as Kim Dae Jung put it in a speech on Sunday.

By implication and innuendo, Kim Young Sam echoes the charges of the ruling party about his rival - that Kim Dae Jung is a relative extremist whose candidacy will polarize politics. He depicts himself as a leader commanding ``support from all sectors of Korean society,'' particularly from people ``who are looking for a stable transfer of power.''

Kim Young Sam is also quick to dispute Kim Dae Jung's image as a dissident and martyr for democracy. While Kim Dae Jung was in exile in Japan and the US for some of the last 15 years, Kim Young Sam says, ``I was in Korea all the time'' fighting the dictatorships.

He says he was responsible for events from the downfall of Park Chung Hee in 1979 to the demonstrations of this summer. ``Because of those activities, people have in their mind that I am a freedom fighter for democracy; therefore I will get the most votes.''

Besides, the silver-haired politican argues, Kim Dae Jung's candidacy will inflame regional antagonisms. Kim Dae Jung hails from the traditionally backward and antigovernment Cholla region in the west. Kim Young Sam and Roh Tae Woo hail from the more populous eastern region of Kyongsang.

In many areas of Korean society, the mood is more one of dismay, even disgust, with both men for not putting aside their ambitions. The failure of the two Kims to reach a consensus, Kim Young Sam admits, ``will disappoint the people - if we have a single candidate from our party, then it will have a powerful impact to end the military dictatorship.''

This is not lost on the ruling party. Mr. Roh, says a top aide, ``feels more than a single candidate will split the opposition vote and work to the advantage of the DJP.'' The difficulty, says DJP Deputy Secretary General Hyun Hong Choo, is that all three other candidates ``will direct their attack against Roh.''