South Korea's voters go to the polls to elect a new president Thursday. North Korea's latest security threat and South Korea's ambivalence over its ties with the United States have emerged as the campaign's key issues, with the two top candidates offering different approaches. Rising tensions over North Korea's nuclear weapons and growing anti-American sentiment have turned the election into a poll on the country's relations with its neighbor and the United States.

South Koreans on Thursday will elect a leader to succeed President Kim Dae-jung. The constitution bars him from seeking another five-year term.

The leading candidates are Roh Moo-hyun, a 56-year-old labor lawyer seen as the president's protégé, and former prime minister Lee Hoi-chang, 67, who is well known for his conservative views.

Late Wednesday, popular Korea Football Association president Chung Mong-joon withdrew his support for Mr. Roh, casting doubts on his election prospects. The two had formed a political alliance after Mr. Chung dropped out of the presidential race last month. Mr. Chung complained Mr. Roh made an inappropriate remark about relations between North Korea and the United States during a campaign speech earlier Wednesday.

North Korea has emerged as the election's focus, says Scott Snyder, the Asia Foundation's Seoul representative. "There is a broad public consensus for continuing to have some form of engagement with North Korea. I think there is a desire to see the North Koreans respond with some form of reciprocity. So I think we will continue to see some form of engagement but with tougher terms if Mr. Lee is elected. Mr. Roh appears to prefer a policy that is more in line with the generosity that has characterized the sunshine policy," Mr. Snyder said.

Mr. Roh pledges to press on with Kim Dae-jung's so-called sunshine policy of engagement, aid and cultural exchanges with the communist North, despite Pyongyang's plans to continue its nuclear program.

The United States says North Korea has revealed it has a secret program to enrich uranium, which can be used to make nuclear weapons. It has also announced it will try to restart nuclear power facilities capable of producing plutonium, which also can fuel weapons.

In contrast, Mr. Lee calls the sunshine policy naive. He thinks Seoul has given too much to impoverished North Korea in the past five years, such as food aid and tourism projects. Instead, he says he would offer cooperation only if Pyongyang halts its nuclear programs.

In Seoul, voters' comments underscore the issue's importance to the presidential race, which pollsters say is too close to call.

One 26-year-old man said the Kim government's best achievement is the sunshine policy and he hopes the country's next leader will uphold it.

A 47-year-old school principal said Mr. Lee will wisely resolve the North Korea problem and added that she believes the North Korean government will not act recklessly either.

Also dominating this election is South Korea's relations with the United States, which bases 37,000 troops in the country, most of them near the Demilitarized Zone that separates North and South Korea. The two countries remain technically at war since the Korean War ended 49 years ago with no peace treaty.

Anti-American sentiment has been present for decades, but emotions are at a new height after a U.S. military court last month acquitted two soldiers in the deaths of two South Korean girls in a traffic accident.

A crowd of about 30,000 protested the verdict at Seoul City Hall on Saturday night. They demanded that South Korea's next president renegotiate the pact governing the conduct of U.S. soldiers in the country.

Both Mr. Roh and Mr. Lee have said they advocate rewriting the agreement, but they also say they want to maintain good ties with Washington. Mr. Lee is seen as having close ties to the Bush administration, while Mr. Roh, who has never traveled to the United States, is widely perceived as more independent.

Despite these concerns looming over the electorate, up to 20 percent of the voters say they remain undecided. Political science Professor Lee Chung-min of Seoul's Yonsei University said many voters are grappling with South Korea's role on the world stage.

"The election we will see in a few days comes down to this: what type of future do Koreans want for themselves? Do we want a Korea that is strongly aligned with the West, which has a truly global economy and a global information infrastructure and values that are universal, such as democracy and human rights and so forth. Or do we want a Korea that is more insulated, isolated, more anti-America and more inward looking? So whether or not they support U.S. forces [in Korea] most will argue that the Korea of the 21st century looks outward, not inward," the professor said.

The Asia Foundation's Scott Snyder notes that the serious issues facing voters are creating a wide social gap. "This is a referendum on the sunshine policy but also on Korea's future. One of the aspects of the Korean election that is most interesting is the emergence of a generational divide that is going to be tested in this election, so in some ways it is also a referendum about stability versus aspiration or hope for the future," Mr. Snyder said.

In general, younger voters are expected to rally behind the progressive Mr. Roh, while older citizens will support conservative candidate Lee.

The winning candidate will face unprecedented challenges in the nation's international ties, both with its fiercest rival, North Korea, as well as its key ally, the United States.