SEOUL -- They came from opposite sides of South Korea to work in a Seoul clothing factory and, like thousands of newcomers in the big city before them, they fell in love.

When the young woman's parents, from the poorer but proud southwest, discovered that her boyfriend came from the east, they insisted that she break their engagement. Her boyfriend checked into a Seoul inn last month and took a fatal dose of poison.

The Korean Romeo and Juliet story resonated throughout this country, where virulent regional prejudice looms as a possibly decisive factor in the first democratic election in 16 years, scheduled for next month. Charismatic opposition leader Kim Dae Jung, from the disadvantaged southwest, is battling two candidates from the more affluent southeast in a contest that some fear could heighten the nation's debilitating regional biases.

Regionalism explains why Kim may have a better chance in a four-way race, even with a split opposition vote, than if he alone represented antigovernment forces. Kim and rival opposition leader Kim Young Sam risk splitting the antigovernment vote -- but it may matter more that Kim Young Sam and government candidate Roh Tae Woo will split the vote of their Yongnam region. (The fourth candidate comes from a small central province, neutral in the regional wars.)

"The single most important predictor of voting behavior, overwhelmingly, is not age, not education, not ideology, but what region you come from," said one political insider with access to reliable public opinion polls. Publication of opinion polls is banned in South Korea.

Tensions between natives of Kim Dae Jung's Honam and Kim Young Sam's Yongnam regions, often overlooked by outsiders, are as strong as ethnic prejudices in many other nations. They color Koreans' personal relations, their work life and their politics.

"Regionalism now represents the most serious weak point for the future of our politics and our country," a South Korean diplomat said.

Honam, which literally means "south of the lake," includes the historically disadvantaged southwestern provinces of north and south Cholla and the independent city of Kwangju. Yongnam, which means "south of the mountain," includes the more prosperous southeast provinces of south and north Kyongsang and the cities of Pusan and Taegu.

Distrust goes back 1,000 years, to a time when the peninsula was divided among three kingdoms. All Koreans know the centuries-old stereotypes: Yongnam natives are stubborn, honest and slow; Honam folk are quick-witted but mercurial and deceitful.

Japan exacerbated the divisions with its colonial rule during the first half of this century, developing the southeast's port cities, which faced Japan, and connecting them by railroad to the industrial north. Yongnam flourished, while the flat and verdant southwest, once prosperous as the nation's rice bowl, lagged.

But it was under the two military-installed regimes of the past 26 years, many Koreans say, that regional prejudice degenerated from manageable rivalry to festering anger. Park Chung Hee, who took power in a 1961 coup and ruled until his assassination in 1979, and current president Chun Doo Hwan, who staged his own coup after Park was killed, both came from Yongnam. Development there took off while the rural Honam provinces were neglected.

As a result, population in the southwest declined from 6.5 million in 1966 to 6 million 20 years later, while the southeast's population grew from 7.5 million to 12 million during the same period. Many young people left Honam for the poorest paying jobs elsewhere in the developing nation. Teen-age girls from Honam filled sweatshops or, for a roof and a daily ration of rice, worked as maids. Their bosses often came from Yongnam, fueling resentments. Young men from Honam still disguise their accents when applying for jobs.

Even within Honam, high-ranking government officials, generals and factory managers are likely to hail from the southeast. In the Army, a saying goes, Honam recruits get beaten more often just because of their origin. "In Seoul, the immigrants from Kyongsang have the top positions," said former student leader Lee Shin Bom. "The people from Cholla are at the bottom."

The government releases few statistics on the comparative well-being of the two regions. But to drive into rural Cholla, following highways until they give way to mud tracks, is to glimpse the Third World poverty that much of the nation has left behind.

As South Korea prepares for next month's election, attention is focusing on the issue of regional prejudice. Each candidate presents himself as the one who can heal what Kim Dae Jung called the "shameful and ruinous" provincial prejudice, while each accuses the others of inflaming regionalism.

The two Kims, longtime rival opposition leaders, are in many ways regional rivals who agree on most policies. Virtually all of the National Assembly members loyal to Kim Young Sam come from Yongnam, and almost all of Kim Dae Jung's from Honam.

During the summer when the Kims were promising to unite behind one candidate, Kim Young Sam's aides argued that he should run because his region is home to so many more voters. Now he says Kim Dae Jung's candidacy will worsen prejudices.

Kim Dae Jung, for his part, argues that his regional origin should not be held against him and criticizes Kim Young Sam for raising the issue. Privately, his followers say it is past time for their region to be represented in the Blue House, South Korea's presidential mansion.

"Frankly speaking, as a Honam person, I would like to see a president from here," a college sophomore majoring in economics said at a recent rally in Kwangju.

Kim Dae Jung also says that only he can heal the wounds of the Kwangju insurrection, when soldiers gunned down hundreds of Honam civilians protesting Chun's coup in 1980. Only Kwangju, the spiritual heart of Honam, actively resisted the coup, and many Kwangju residents have never accepted government denials that troops brought in to quell their peaceful protest were deliberately recruited from Yongnam so that they would respond more viciously.

Roh Tae Woo, the ruling party candidate from north Kyongsang in Yongnam, recently traveled to Kwangju and prayed before the grave of a Roh who he said was his Honam ancestor 57 generations back. After offering fruit, dried fish, rice wine and incense before the grave, Roh said, "I felt like my founding grandfather was telling me, 'Your ancestors are buried all over the country, so I hope you will do your best to eliminate regional fighting as much as possible.'" Despite what he said were his ties to Honam, Roh received a generally hostile reception during his campaign tour through the region.

The fourth candidate, former prime minister Kim Jong Pil, offers himself as a solution to regionalism because his home is neither Honam nor Yongnam. But Chungchong province has only 2.6 million voters, and, partly for that reason, the third Kim is an underdog.

The numbers, however, defy easy predictions. If Roh and Kim Young Sam split the 7.6 million votes in their area and Kim Dae Jung wins most of the 3.5 million in his -- allowing for some crossovers in each case -- the election could be decided in Seoul and its environs, now home to 10.6 million voters. Like Washington, D.C., Seoul is a city of immigrants, with perhaps one quarter who hail from Kim Dae Jung country, one-third from Yongnam and the rest divided among refugees from the north, settlers from smaller provinces and Seoul natives.

How many of Seoul's residents, half of whom now think of themselves as middle class, will be motivated by something other than regional loyalty is uncertain. But the Korea Herald, editorializing about the clothing factory worker's suicide, was not optimistic. "We wish it were an isolated incident," the English-language daily newspaper wrote about the lover's death, "but developments in the political arena and in many other areas of society do not bode well for an end to the unfounded, though long held, provincialism that undermines the integrity of the nation."