How to manage “the Trump risk,” as local news media put it, has become a major campaign issue. The candidates are falling over themselves to show they would be the best at handling him, either by drawing him closer or by being tougher.
“National security has become a dominant election issue this year thanks largely to Trump,” said Koh Yu-hwan, a professor of North Korean studies at Dongguk University in Seoul. “Whether he intended it or not, Trump ended up intervening in the South Korean election.”
At the least, many South Koreans fear a rift in the close relationship with Washington. At worst, some fear Mr. Trump could do something impulsive, like ordering a military attack on North Korea, with little regard for the devastation it would cause here in the South.
The leading conservative candidate, Hong Joon-pyo, sought to calm jitters by swearing that if he is elected, he will hold a summit meeting with Mr. Trump aboard the Carl Vinson, the aircraft carrier Mr. Trump recently dispatched to Korean waters in a show of force against the North.
Ahn Cheol-soo, a centrist, has pushed what he sees as his advantage over other candidates: He and Mr. Trump both went to the Wharton School at the University of Pennsylvania. He says that connection will help him build rapport with Mr. Trump.
Moon Jae-in, the Democratic Party candidate, who surveys project will be the winner, has also vowed to meet Mr. Trump in one of his first acts as president. But he indicated that he would be more confrontational. He has said that instead of avoiding friction with Washington, South Korea should “learn to say no.”
He and many South Korean voters have been outraged by Mr. Trump’s recent suggestion that their country pay $1 billion for a missile-defense battery the United States installed last week. Mr. Moon told a cheering crowd at a campaign rally this week, “Which candidate can do a proud diplomacy, saying what we need to say to the Americans?”
Mr. Moon represents liberal political forces in South Korea who say that under its conservative and pro-American leaders, the country’s alliance has become too “hierarchical” and too “tilted” in Washington’s favor. Liberals have also called for dialogue with North Korea, saying that Mr. Trump was wasting his time if he was waiting for China to enforce United Nations sanctions forcefully enough to stop the North’s nuclear program.
Mr. Moon seeks to bring back a version of the “Sunshine Policy,” under which South Korea promoted dialogue and economic exchanges with the North from 1998 to 2008 to help build trust and lessen Chinese influence on the northern half of the divided peninsula.
Those on the left view the Trump administration’s talk of a “military option” against the North as dangerous. They also do not want their country dragged into a hegemonic struggle between the United States and China, its biggest trade partner.
Mr. Trump’s rough diplomacy could shake a delicate balance in northeast Asia, prompting South Korea to redefine its alliance with Washington and move closer to China, analysts say.
“Washington has grown used to conservative governments in Seoul that back a hard-line approach to the North, so it could be an awkward and painful period of adjustment in getting used to liberals’ very different philosophy of inter-Korean relations,” said John Delury, a professor at the Graduate School of International Studies at Yonsei University in Seoul.
South Korea’s alliance with the United States has gone through some rough patches, as when Presidents Richard Nixon and Jimmy Carter threatened to pull out American troops and when violent anti-American protests rocked college campuses in the 1980s and beyond.
But South Koreans have never dealt with an American president like Mr. Trump, who appears to question the relationship. How well their leader gets along with Washington tends to affect their sense of security.
“Trump is unpredictable like a rugby ball, and that’s adding uncertainty to the alliance,” said Lee Byong-chul, a senior fellow at the Institute for Peace and Cooperation in Seoul.
Mr. Delury said that for South Koreans, who were “used to the United States playing a stabilizing and benevolent role,” the Trump presidency was “uncharted territory.”
“The combination of Donald Trump as the American president and a progressive as South Korea’s president may be toxic.” said David Straub, a former American diplomat and the author of the book “Anti-Americanism in Democratizing South Korea.”
During his campaign, Mr. Trump irked South Koreans by repeatedly accusing them of getting a free ride in their national defense and not paying enough for the American military presence here. After his election, he sent top aides, including Secretary of State Rex W. Tillerson, to reaffirm that he was with the South Koreans “100 percent.”
But he also hurt their national pride by saying Korea used to be “a part of China.” Some accused Mr. Trump of ignoring their country when he discussed North Korea with President Xi Jinping of China and Prime Minister Shinzo Abe of Japan on April 24 but called no one in Seoul.
Last month, Mr. Trump’s administration stepped on a hot-button election issue in South Korea by pushing ahead with the deployment of the Terminal High Altitude Area Defense, or Thaad.
Conservative candidates like Mr. Hong called for an early deployment to guard against North Korean missiles, even though China threatened retaliation against South Korea if it allowed the powerful American radar on its doorstep.
Mr. Moon repeatedly warned that Washington should postpone the deployment to give the next South Korean government time to review it, as his liberal supporters accused the United States of foisting the system on them.
Last week, not only did the United States push ahead with the deployment, but Mr. Trump also said South Korea should pay for Thaad, contradicting an earlier agreement that Washington would pay for the system and Seoul would provide the land and basic infrastructure. (On Sunday, the White House national security adviser, Lt. Gen. H. R. McMaster, called his South Korean counterpart, Kim Kwan-jin, and “the two reconfirmed what has already been agreed” about the system’s costs, Mr. Kim’s office said in a statement.)
Mr. Trump’s Thaad comment was also a huge letdown for conservative South Koreans who supported the deployment, even arguing that a boycott of South Korean goods and other retaliations from China were a price South Korea should pay willingly to protect the alliance with the Americans.
“Our fear that Trump treats everything as a money deal and in terms of gain and loss has become a reality,” the leading conservative daily, Chosun Ilbo, said in an editorial over the weekend, reporting the “shock, betrayal and anger many South Koreans have felt.”
In a news conference this week, Song Young-gil, Mr. Moon’s main campaign adviser, was more blunt: “Because of our obsequious diplomacy over Thaad, the Chinese are retaliating against us and the Americans have stabbed us in the back.”