South Korean President Moon Jae-in will meet with President Trump in Washington on Thursday in an effort to jump-start stalled diplomatic talks with North Korea. It will be his first face-to-face meeting with Mr. Trump since February's inconclusive summit in Hanoi.

The whirlwind, one-day trip is of critical timing and pivotal importance to Moon, who faces record-low domestic approval ratings – now below 50 percent from a high of nearly 90 percent – and the specter of consequential legislative elections next April.

Although Mr. Trump has said that he is "in no rush" to see North Korea denuclearize provided it does not resume missile testing, Moon is staring down a number of symbolic dates by which the success of his administration's overtures to the North is likely to be judged.

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April 27 will mark the one-year anniversary of Moon's first meeting with North Korean leader Kim Jong Un and the first inter-Korean summit in over a decade. The second-year anniversary of Moon's inauguration is on May 10. The first Trump-Kim summit in Singapore took place on June 12, and the first anniversary of Moon's trip to Pyongyang follows in September. Each one serves as a meaningful benchmark in negotiations with the North – and an opportunity for renewed public criticism.

"This has to be a really speedy process," said one senior South Korean government official who requested anonymity to discuss sensitive topics. If Moon is politically crippled, the official said, "then this process will be doomed."

Hanoi fallout

Government officials in the U.S. and South Korea have spent the last several weeks amplifying the positive outcomes of Hanoi, which, those officials argue, include a better understanding of the respective parties' top demands. The summit ended early and abruptly after the U.S. demanded North Korea's wholesale nuclear and WMD disarmament while rejecting Kim's counteroffer to dismantle Yongbyon, a sprawling nuclear complex thought to be the heart of the North's nuclear program, in return for lifting painful United Nations sanctions.

"What it comes down to is the formula that the two sides need to negotiate for the next round of the game," the senior South Korean official said. "So in other words, they now know what to negotiate."

Still, the Blue House -- the South Korean equivalent of the White House -- appeared to have been blindsided by the collapse of the talks.

"We thought Hanoi would produce some kind of agreement. We never thought that summit diplomacy would go wrong," said Yonsei University professor Chung-in Moon, who serves as a special advisor to President Moon but stipulated in an interview that he was not speaking for the government. "I think the Blue House was shocked."

Chung-in Moon and other South Korean officials and experts spoke to visiting reporters from several U.S. outlets, including CBS News, as part of the Korea Journalist Fellowship Program. The program was sponsored by the Atlantic Council, a nonpartisan think tank in Washington, in partnership with the Korea Foundation.

Chung-in Moon added that there was a "widespread understanding" in Seoul that National Security Advisor John Bolton played a "very, very negative role" in derailing the negotiations in Hanoi.

The United States "essentially crushed North Korea with their maximalist position" by including demands for Pyongyang's biological and chemical weapons, the senior South Korean government official said. "[I]t presents us with a challenge."

A senior U.S. administration official defended the outcome in Hanoi, telling CBS News, "The president makes the policy, and the president made the right decision to leave Hanoi without a deal."

Although Seoul was wholly unprepared for a no-deal scenario, Mr. Trump called President Moon from Air Force One immediately following the Hanoi summit and urged him, multiple times, to call Kim directly – and then come discuss next steps in Washington, according to the senior official. The Blue House said at the time that Trump had asked President Moon to "perform the role of a mediator."

Though it is unclear whether the Moon-Kim call ever happened, South Korean officials say numerous communication channels, including via Russia and China, remain open with the North.

"We are doing a lot of things to prevent North Korea from walking out of this process," the government official said. "[S]ince the Hanoi summit we've been saying, 'You guys need to calm down; don't do anything stupid from [rocket launching site] Tongchang-ri.'"

Easing sanctions

"I think the basic difference between Seoul and Washington is how to look at sanctions," said Jae-Ho Chung, a professor of international relations at Seoul National University. "Basically, the Blue House regards mitigating sanctions as an incentive to bring North Korea to the negotiation table; on the other hand, the White House looks at [their] sustenance as a measure to pressure Pyongyang to denuclearize," Chung said.

Since Hanoi, U.S. officials have struck a mostly optimistic tone about the future of talks with Pyongyang while holding firm to the administration's stated policy of maintaining maximum economic pressure until the Korean peninsula is fully and verifiably denuclearized.

The senior U.S. administration official said the United States had "clearly defined the scope of North Korean WMD and ballistic missile programs and have a shared understanding of what final, fully verified denuclearization entails and what meaningful progress toward that goal looks like."

"Unfortunately, the North Korean position fell far short of that understanding," the U.S. official said.

Last week, Secretary of State Mike Pompeo told CBS News he was "confident" there would be a third summit with Kim, but said President Trump had been "unambiguous" in his position that no sanctions would be lifted until denuclearization was achieved.

On Wednesday, Pompeo seemed to soften that stance in congressional testimony, telling senators on the Foreign Relations Committee he wanted to "leave a little space" on the question of easing sanctions.

"From time to time there are particular provisions that, if we were making substantial progress that one might think, you know, that was the right thing to do," Pompeo said. He added that "the core UN security council resolutions need to remain in place until the verification of denuclearization has been completed."

Any opening would be a welcome development for President Moon who, during his visit on Thursday, may try to thread a diplomatic needle by asking the United States to "tolerate" some inter-Korean economic coordination, especially if Pyongyang puts something new on the table.

"I personally would argue that North Korea should take some concrete actions," like allowing American experts to inspect Tongchang-ri and nuclear testing site Punggye-ri, Chung-in Moon said. "Then the U.S. could make some kind of relaxation of sanctions."

In his interview with CBS, Pompeo declined to say whether the administration would be open to permitting limited economic exchanges between North and South, including reopening the Kaesong Industrial Complex and permitting tourism at Mount Kumgang, both sites of past inter-Korean cooperation.

"I understand the sentiment but they've been great partners," Pompeo said of his South Korean counterparts, "and we have worked closely together to enforce these sanctions."

"We appreciate what they're doing," Pompeo said.

The sequencing of sanctions relief also appears to be at issue. The Moon administration is attempting to merge the United States' demands for a comprehensive, "big deal" agreement with the North Koreans' demands for a step-by-step, "small deal" approach, according to Shin Beomchul, the director of the Center for Security and Unification at the Asan Institute for Policy Studies.

"The problem with Hanoi was that North Korea didn't show its interest in denuclearization – so we need some kind of guarantee that the North will end up there," Shin said. "In my view, a roadmap to the end state of denuclearization is necessary."

It is one reason why the Blue House has been forecasting efforts to bring Mr. Trump around to accepting a so-called "good enough" deal, Shin said, one in which the North agrees to a well-defined, timebound end state that involves complete denuclearization for the lifting of sanctions – an "all-for-all" agreement – plus a roadmap for its incremental implementation.

That kind of deal would also facilitate what officials in Seoul are calling an "early harvest" – understood to be unspecified but achievable wins for both Washington and Pyongyang.

That leaves President Moon with the charge of persuading Kim to sign a comprehensive agreement – something for which he demonstrated little appetite in Hanoi – while also getting President Trump to agree to take incremental steps toward the big deal his administration has demanded.

"Therefore, he has a Herculean task," Chung-in Moon, the special adviser, said.

This post has been updated with quotes from a senior U.S. official.