Mr. Moon has acted as a go-between for Mr. Trump and Mr. Kim. The American president is likely to seek reassurances from Mr. Moon that the diplomatic overture North Korea initiated in January will not end in failure.
Mr. Moon had his own history-making encounter with Mr. Kim last month, which buoyed hopes for the meeting between the North’s leader and Mr. Trump. But last week, the North Koreans said they were having second thoughts, objecting to the Trump administration’s demand that they relinquish their nuclear weapons before receiving any benefits.
Mr. Moon has taken the lead role in orchestrating the diplomacy that led to the Trump-Kim summit. South Korean officials conveyed the invitation from Mr. Kim to Mr. Trump to meet, which the president accepted on the spot, surprising his visitors as well as his own advisers.
Mr. Trump spoke to Mr. Moon by phone on Saturday, suggesting the depth of uncertainty he feels about the change in tone from Pyongyang. North Korea objected particularly to John R. Bolton, the new national security adviser, who said he viewed Libya as a template for negotiating with North Korea.
Mr. Trump subsequently disavowed Mr. Bolton’s remarks, acknowledging that Libya’s voluntary disarmament in 2003 did nothing to protect its leader, Col. Muammar el-Qaddafi, from being killed by his own people in the Arab upheavals less than a decade later.
Speaking last week, the president offered assurances to Mr. Kim that if he agreed to give up his nuclear arsenal, he would survive in power and his people would prosper. But the tone in North Korea’s media has remained suspicious and grudging.
Mr. Moon will likely seek assurances of his own — not least that the United States will maintain American troop levels in South Korea, regardless of its negotiation with Mr. Kim. Mr. Trump has long expressed a desire to withdraw troops, and the National Security Council has asked the Pentagon to prepare options for changing levels of military forces.