Some analysts have warned that Seoul's security alliance with Washington might crumble if the next US president were Mr Donald Trump, who has said he might withdraw the 28,000 US forces stationed in South Korea to keep North Korea at bay.
The presumptive Republican nominee has also demanded that Seoul pay the full cost of keeping US troops here, instead of about half the bill or 920 billion won (S$1 billion).
Sogang University's political science professor Kim Jae Chun said "no US President can single-handedly scrap the alliance", but ties would be "adversely affected" under a Trump presidency.
There are also worries that Mr Trump may implement protectionist trade policies that will hurt South Korea's exports to the US, and that he might push for South Korea and Japan to develop their own nuclear weapons against nuclear-armed North Korea, which could lead to a nuclear arms race in East Asia.
In contrast, South Koreans view Mr Trump's key rival, Democratic front runner Hillary Clinton, more favourably as they are more familiar with her work as US Secretary of State from 2009 to 2013. Mrs Clinton visited Seoul five times during her tenure, including in 2010 to pay tribute to 46 sailors killed in a warship sinking blamed on the North.
Prof Kim said Mrs Clinton is well versed in foreign policy and more likely to "stay committed to the US-Korea alliance". However, there is also concern that her hawkish stance on North Korea could rock stability in the Korean peninsula.
"If Trump is an earthquake, Clinton is a storm. Either way, Korea is faced with a serious challenge," said an editorial in the JoongAng Ilbo newspaper.
Chang May Choon