Moon Jae-in on the campaign trail earlier in 2017. Photo: Reuters

South Korean liberal presidential candidate Moon Jae-in’s polling lead over his nearest rival Ahn Cheol-soo has strengthened ahead of Tuesday’s election, setting up a dramatic victory in a generations-long political battle.

Moon was arrested in the 1970’s while protesting recently impeached President Park Geun-hye’s father, former President Park Chung-hee. Decades later, Moon lost a tightly contested 2012 presidential race to Park Guen-hye.

The likely victory for Moon poses a challenge for the Trump administration as the US is increasingly reliant on the South to pressure North Korea and balance China’s growing influence. Moon represents a more dovish stance toward North Korea, and has spoken out against the deployment of the THAAD missile defense system.

The missile defense system, as is widely known, was deployed for broader regional strategic goals of the US, not for the defense of South Korea. Despite this, and despite widespread South Korea public opposition to its deployment, Trump linked South Korean payment for THAAD with bilateral trade policy.