SEOUL, OCT. 26 -- Aides to Kim Dae Jung said today that the South Korean opposition leader has decided to set up his own political party and run for president as its nominee, breaking away from the party headed by Kim Young Sam.

The move would formalize the long-predicted split between the two Kims and mean that both men will be running for president, barring a last-minute compromise. It follows nearly two months of failed talks between the longtime rivals over selecting a single opposition nominee. The two-Kim candidacy is expected to divide the opposition vote, significantly increasing the electoral chances of Roh Tae Woo, the ruling party's nominee.

Kim Dae Jung is to formally announce his plans at a press conference on Wednesday.

His decision was announced today after one of his top aides, Lee Chong Jae, met with Kim Young Sam and demanded that he withdraw from the presidential race.

Kim Young Sam, who announced his own candidacy several weeks ago, refused and reportedly recalled his offer to have a vote showdown at a Nov. 5 convention of the opposition Reunification Democratic Party. Because Kim Young Sam, president of the party, could probably count on more support among party delegates, Kim Dae Jung has turned down the offer, describing it as inappropriate.

Aides to Kim Dae Jung said the opposition leader and his party supporters will boycott the convention. The aides added that Kim Dae Jung's supporters in the party -- reportedly about 30 of its 70 National Assembly members -- would switch their political affiliation once the new party is established.

Publication of public opinion polls on the presidential race is banned, so it is impossible to know with any precision where the candidates stand with voters. However, many political analysts here believe that Roh, a former general, would probably lose in a one-on-one contest but could win if both Kims are in the race. Roh may also benefit from a backlash by voters dismayed with the Kims' bickering and failure to honor a longstanding promise to agree on a single opposition nominee.

The election, to be held by Dec. 20, will be the country's first direct presidential poll since 1971, when Kim Dae Jung narrowly lost to the then-President Park Chung Hee in a contest clouded by alleged government vote fraud. The December election was called after massive protests in June forced President Chun Doo Hwan to agree to scrap the country's current constitution, replacing it with a new charter that mandates direct elections, among other democratic reforms.

The country's 25 million voters are to go to the polls Tuesday in a national referendum on the new constitution. It is expected to receive overwhelming approval.

Referring to the December election, one Kim Dae Jung aide said: "Nobody knows who will win except God, but we think we will win."

Kim Young Sam was quoted in local news reports tonight as saying that Kim Dae Jung would "betray the people's will" by setting up his own party and running as its presidential nominee. But another Kim Dae Jung aide told reporters, "With the failure of all efforts to agree on a single opposition candidate, we have decided that a new party is the best alternative."

Kim Dae Jung has strongly hinted in the past few weeks that he planned to run for president, although it was not clear whether he would do so as an independent or as a candidate of the Reunification Democratic Party. The 63-year-old dissident leader claims his candidacy is justified because of his record of opposing South Korea's military-backed governments, which have jailed him, exiled him and even tried to kill him.

He also says the time has come for a native of Cholla, his home region and an economically disadvantaged area whose inhabitants have largely been excluded from power, to lead the opposition in the elections.

Kim Dae Jung is regarded as a forceful, even fiery politician who can draw large crowds. But he is also deeply distrusted by the country's powerful military generals, most of whom view him as a dangerous radical. Many political analysts in Seoul suggest that hard-line elements of the military might be tempted to step in if Kim Dae Jung were elected president.

Over the past two months, the Kims have jockeyed for position by staging mass rallies throughout the country. But the competition seems to have ended in something of a political draw. Kim Dae Jung drew several hundred thousand people to a homecoming rally last month in Kwangju, the Cholla capital. Kim Young Sam staged a similar homecoming rally in his native city of Pusan and also drew a large crowd.