The U.S. plan to isolate North Korea is likely to get a lot more complicated after Tuesday.

That's when South Koreans go to the polls, and the front-runner for president is a man who promises to give North Korean leader Kim Jong-un a handshake, maybe even a hug.

Moon Jae-in was out practising just that over the weekend. A campaign event saw him giving enthusiastic hugs to many of his supporters on a busy Seoul street.

Students, babies, mothers, fathers, grandparents, the unemployed — they all got a warm embrace from Moon.

So why not North Korea as well, suggests supporter Lee Mi-un.

"I want him to go negotiate well with Kim and defuse the tense situation," she says. "To create a happy situation. And ultimately to contribute towards a peaceful unification of North and South Korea."

Much of Moon's backing is from younger South Koreans like these, born long after a war that divided a people and a peninsula. And with almost 40 per cent support in the polls — double that of his nearest rival — Moon seems set to win.

But his would be a sharp turn from South Korea's current tough approach, and it could put him on a collision course with the country's biggest military backer, the United States.

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Moon's friendlier approach to North Korea could put him on a collision course with South Korea's biggest military backer, the United States, writes CBC's Saša Petricic. (Kim Kyung-Hoon/Reuters)

Washington is ramping up military, economic and diplomatic pressure on North Korea, and insisting that other countries do the same.

It wants to thwart Kim Jong-un, who has vowed to develop a nuclear warhead with which to hit the U.S. mainland. His missile program has made huge strides in the past year, despite frequent accidents and failures.

The U.S. has now even threatened a pre-emptive strike to stop him.

On the other hand, Moon — a liberal lawyer whose parents fled the North during the Korean War — has been proposing something very different.

His policy is to revive a so-called Sunshine Policy from 20 years ago that stressed neighbourly co-operation over confrontation. At that time, South Korea wooed the North with billions in humanitarian and economic aid in a failed effort to prevent its weapons program from gathering momentum.

Moon's version has been dubbed Sunshine 2.0.

It advocates reopening a joint North-South industrial zone, closed last year after North Korea tested missile after missile in defiance of UN demands that it stop. Moon has suggested the rebirth of the Kaesong zone could even be the first step in a long-term plan to reunify the economies of the two countries.

In the short term, it would provide North Korea with much-needed foreign cash, perhaps contrary to tough international economic sanctions mandated by the UN Security Council. And it would certainly go against the efforts of U.S. Secretary of State Rex Tillerson. Before the Security Council last month, he predicted "catastrophic consequences" unless all countries "put new pressure on North Korea to abandon its dangerous path."

Moon accused of 'holding hands' with North 

Moon's opponents have accused him of planning to dismantle six decades of U.S.-South Korean military co-operation, an alliance they say the country needs more than ever in the face of an armed and dangerous North.

"He wants to give North Korea billions," said Hong Joon-pyo, candidate for the conservative Liberty Korea Party at a rally in Incheon on Saturday. "And why? So they can build more weapons and threaten us with our own money."

Hong's party was initially written off in this election, abandoned by most voters because of its connection to the former, disgraced president, Park Guen-hye. She was impeached in March and now sits in jail, facing corruption-related charges.

But North Korea's continued missile tests and threats have frightened many South Koreans, especially older ones, and Hong has experienced a modest surge.

"Honestly, I'm worried," says Kim Myung-sook, a retiree from Incheon who stands with a Hong banner. "We are at the brink of war. When I open my eyes in the morning, I think to myself, 'I have survived another day.'

"And that Moon? He's holding hands with the North," she says.

Mixed signals from Trump

Another conservative candidate, People's Party founder Ahn Cheol-soo, has also benefited from that anxiety. He is in second place and analysts say he has the potential to surprise.

The other unpredictable element in the campaign has been mixed signals coming from U.S. President Donald Trump.

He has sent Tillerson and others to assure South Koreans that he is with them "100 per cent," but at the same time, he has questioned their commitment and contribution to their own military protection, suggesting they should pay more for the thousands of U.S. troops stationed here.

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Other contenders for the top job in South Korea's election are Hong Joon-pyo, right, from the Liberty Korea Party, and Ahn Cheol-soo, with the People's Party. Both have benefited from fears about North Korea's recent missile tests. (Kim Hong-Ji/Reuters)

Trump outraged many by demanding a billion dollars for the deployment of a controversial missile defence system known as THAAD, Terminal High Altitude Area Defense. The demand was later withdrawn by Washington and the system became operational last week.

But the damage was done and Moon benefited from it.

Shin Yul, a political science professor at Seoul's Myungji University, calls this unforeseen intrusion by the U.S. the "American wind."

"The wind from America amplifies the tension on the Korean peninsula. And this brings the issue of national security at the forefront of the election discourse. This is very unusual."

Moon himself has softened some of his rhetoric, now that he may have to deal with Trump if elected president.

Moon won't take 'back seat' to U.S.

In an interview with the Washington Post last week, Moon said, "The alliance between the two nations is the most important foundation for our diplomacy and national security."

Moon promised to work together with the U.S on the North Korean nuclear issue, but said he would not "take a back seat" to U.S. decision-making.

"I believe we need to be able to take the lead on matters in the Korean peninsula as the country directly involved."

North Korea has been uncharacteristically quiet about South Korean affairs during the campaign. But this week, an editorial in the ruling Workers' Party newspaper Rodong Sinmun seemed to endorse Moon's conciliatory approach.

It said that could "resolve the layers of misunderstanding and distrust," so long as Seoul would "distance itself from the disgraceful and servile U.S. policies."