Deepening internal strife and a series of political blunders are throwing the campaign of the leading presidential candidate from South Korea's opposition into disarray, sending his poll numbers nose-diving and giving a boost to rivals.
Housing prices and other domestic issues are expected to influence voter sentiment, but the election would shape Seoul's balance between the United States and China, and how South Korea coordinates with Washington on North Korea.
Yoon, a former prosecutor-general and political novice, has held a tight lead in opinion polls for months, capitalizing on the disillusionment with President Moon Jae-in's policy failures and what voters see as hypocrisy among political leaders.
Yoon's ratings have fallen in recent weeks, as he has failed to present a policy vision and mediate in increasingly vitriolic infighting in his party. He has been caught up in a controversy involving his wife.
Lee has seen his popularity surge, overtaking Yoon in most polls since last week despite a scandal over his son's illegal gambling. According to a poll released Monday by Global Research, 37% of respondents supported Lee, and 28% were for Yoon.
The infighting in the People Power Party took a new turn when senior members of Yoon's campaign team offered to resign on Monday amid disagreements over replacing some of them, prompting Yoon to cancel his schedule for Tuesday to take time to think.
Yoon told reporters late on Monday that I am fully responsible for making many people worry about the election, and that is because of my shortcomings.
Yoon's perceived shortcomings have also prompted the rise of Ahn Cheol-soo, a renowned software mogul and opposition challenger who lost to Moon in a 2017 election.
According to the Global Research survey, more than 41% of South Koreans chose Ahn as the more appropriate contender and around 31% chose Yoon, if both camps agree to unite under one candidate.
Yoon's campaign has said its priority is to increase support from young voters, not Ahn, while Ahn has said he is willing to meet anyone but not considering a unified campaign.
Lee, who had previously sought to joint hands with Ahn, said on Monday that Ahn s moves would be important but warned against any potential collusion between Ahn and Yoon.