Vice President Pence warned North Korea not to test President Trump during a press conference in South Korea on April 17, citing "the strength and resolve of our new president in actions taken in Syria and Afghanistan," as examples. (The Washington Post)

SEOUL – The Trump administration is planning to drastically ramp up pressure on North Korea, and it needs cooperation from America’s partners in the region for the strategy to work. But deep uncertainty about the future of South Korea’s government could undermine Donald Trump’s plan to tighten the noose around the Kim Jong Un regime.

Vice President Pence toured the demilitarized zone Monday and looked across the border to North Korea. He delivered a tough message promising that “the era of strategic patience is over” and “all options are on the table” to deal with Pyongyang’s nuclear and missile mischief. Pence told CNN that the United States plans to “redouble our efforts to bring economic and diplomatic pressure” on North Korea, in conjunction with allies and hopefully China as well.

Pence then met with the acting president of South Korea, Hwang Kyo-ahn, and National Assembly Speaker Chung Sye-kyun, both of whom echoed Pence’s tough talk on how to deal with the North Korean regime. Nobody mentioned during the meetings that both of these leaders will likely not be in power after South Koreans go to the polls to choose a new president next month.

At the exact same time Pence was delivering his speech reassuring South Korea that the United States will stand “shoulder-to-shoulder” with its ally on the peninsula, news broke that the most pro-American leader South Korea has had in a generation, ousted president Park Geun-hye, had been indicted on 13 criminal charges.

The two leading candidates to replace her in South Korea’s May 9 presidential election are Moon Jae-in from the left-leaning Democratic Party of Korea, and Ahn Cheol-soo, a software tycoon who helped found the somewhat more centrist People’s Party. Moon’s proposed policies clash more directly with Trump’s than Ahn’s, but neither is likely to be as close to the Trump administration on key issues as the Conservative Party leaders who will soon leave power.

Both candidates requested meetings with Pence during his two-day stop here in South Korea, but the vice president declined to see them, a White House foreign policy adviser on the trip said.

“We’re able to work with whoever the people of South Korea decide will be their president,” the adviser said, adding that Pence didn’t want to be seen as supporting one candidate or another.

The adviser also acknowledged that the most pressing bilateral defense initiative, the deployment of the THAAD — Terminal High Altitude Area Defense — missile defense system in South Korea, could face changes depending on who wins the upcoming election.

“With the elections coming up, it’s moving. There’s some things to work out. The timeline is a South Korean government decision. It may slip a couple weeks or months,” the adviser said. “Candidly, until they get a presidential election in the first part of May, rightfully so it should be a decision for the next president.”

Another White House official told reporters later that there has been no change in U.S. government policy on THAAD deployment and that the first adviser hadn’t intended to signal one.

Pence reinforced the U.S. commitment to THAAD, standing next to the outgoing acting president, and also criticized China for its campaign of massive retaliation against South Korea for deploying THAAD. The Chinese have been punishing South Korea officials, businesses, tourists and even entertainers as pressure to get the Seoul government to reverse course.

Moon has called for a review of THAAD if he is elected. Ahn has said he supports the deployment, although he might be willing to negotiate it away if China cooperates on the larger North Korea problem. Either way, the “shoulder to shoulder” messaging on the initiative could soon end.

That’s not the only place the next president of South Korea could break with the Trump administration. Moon is pledging a huge expansion of the Kaesong Industrial Complex, a joint North-South economic initiative that was shut down last year. If reopened, that could funnel money to the Pyongyang regime, undermining sanctions.

Moon could also seek early negotiations with the North, in direct contravention of Trump’s plan to exert “maximum pressure” on the Kim regime through increased sanctions and only engaging after North Korea has changed its behavior. It’s no coincidence that Moon’s plan dovetails with the strategy openly endorsed by China.

The Trump administration is aware of the challenge the turmoil in South Korea presents to its North Korea strategy and State Department special representative for North Korean policy Joseph Yun did quietly meet with the candidates on a visit to Seoul last month.

But if Trump administration officials have a plan of what to do if the South Korean government opposes their strategy, they aren’t saying. The Trump team must come up with one quick, or they risk seeing their North Korean strategy end before it starts.