Kim Byoung-Joo, adjunct professor at Hankuk University of Foreign Studies, told CNBC's "Squawk Box" on Tuesday that if the polls are correct and Moon wins a majority of votes, the proportion of votes will be closely watched.
"I think he was leading the polls [with] about 40 percent, and if he gets the votes over 40 percent or not will a very big question because it will really mean a lot about his mandates," Kim said.
"The number of votes will pretty much determine overall the initial course of his presidency," he said.
"If he gets more votes than 40 percent, it will probably mean that he will have a stronger mandate and he will have a greater confidence to push ahead with the policies that he has been talking about," Kim said. "But if he gets anything below 40 percent, he will have to reach out and embrace the center of the political curve, if you will, and he has to be more accommodating of the centralist desire about how flexible he will have to be."
Kim was less concerned about Moon's stance on North Korea, expecting that if he wins, the candidate will need to adjust his policy positions to be more in line with Washington's.
But Goldman Sachs noted that based on opinion polls, if Moon is elected, he could be heading to the Blue House with the lowest electoral support level for any candidate since 1987.
"Should Mr. Moon be elected to the presidency, the win would be the first by a progressive candidate since December 2002, and the first switch to a progressive presidency from a conservative one since December 1997," Goldman said in a note on Friday.
Early voting began on May 4, with local media reports indicating that as much as 26 percent of the electorate may have already cast a ballot.
The country's domestic politics were already in disarray after President Park Geun-hye was impeached and indicted for alleged corruption, with charges including accepting bribes and extorting businesses.
She was forced from office after months of large, peaceful protests.
-- Evelyn Cheng contributed to this article.
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