HANOI — President Trump’s special envoy Stephen E. Biegun arrived here late Wednesday on a mission to close substantial gaps with North Korea ahead of the president’s second nuclear summit next week with Kim Jong Un, a daunting task intensified by skepticism over his approach within the Trump administration.

The challenge for Biegun in meeting his counterpart, Kim Hyok Chol, will be to try to clinch a detailed agreement that can satisfy Trump’s desire for a historic deal but one that can withstand the scrutiny of detractors, including national security adviser John Bolton and others, who warn that Pyongyang is not to be trusted.

Last month, in a lengthy speech at Stanford University, Biegun set out his vision for North Korea to dismantle its plutonium and uranium enrichment facilities in exchange for “corresponding measures” by the United States. Among the incentives Biegun appeared to dangle was a potential peace declaration that would aim to put a formal end to the Korean War, which has been suspended by an armistice since 1953.

Hawks such as Bolton have fiercely opposed this “step-by-step” process in favor of maintaining maximum pressure through economic sanctions that would, in theory, force a better deal by eroding North Korea’s resolve.

Bolton has fretted privately that Biegun’s team is too eager for a deal, and he continues to believe the negotiations will fail, according to people familiar with the deliberations who spoke on the condition of anonymity to discuss the tensions within the administration.

He’s not the only one who is concerned. At a recent interagency meeting, senior officials from the Treasury Department and the Pentagon warned Biegun not to loosen sanctions or move too quickly to agree to an end-of-war declaration, according to a person with knowledge of the talks who spoke on the condition of anonymity to describe private deliberations. The person said it was “startling” to see the concerns raised in the meeting, adding that Biegun has “one job” — to strike a deal — while others in the administration are intent on maintaining a hard line.

Biegun allies outside the administration praised his willingness to press forward on engagement in a difficult environment.

“If you don’t like this approach, then I don’t think you’re in favor of diplomacy — period. And it may be that Bolton isn’t,” said Tod Lindberg, a senior fellow at the conservative Hudson Institute. “This is what good diplomacy looks like.”

White House aides said there was no friction between Bolton and Biegun, but they declined to elaborate on their relationship.

Biegun, 55, a former Ford Motor lobbyist and longtime Republican aide on Capitol Hill, was hired by Secretary of State Mike Pompeo in August to take the reins of the lower-level negotiations after Trump’s first meeting with Kim in Singapore. After the two sides unveiled a thin four-point agreement for reducing tensions and denuclearizing the Korean Peninsula, Biegun was charged with turning the vague statement into a road map to ending the North’s nuclear threat.

Yet Biegun initially struggled to establish himself as North Korean officials dragged their feet in naming his counterpart and sought instead to appeal directly to Trump, who has established a personal rapport with Kim.

The question, people who know Biegun said, is whether Trump has empowered him to drive a hard bargain — or whether his talks with Kim Hyok Chol are mere window dressing for a president who favors his own instincts and improvisational style over careful talking points and studied deliberation.

“The dilemma that Steve and the negotiators face is that the North Koreans view Donald Trump as their pot of gold and they are not going to negotiate” with the president’s subordinates, said Michael Green, an Asia expert at the Center for Strategic and International Studies who worked with Biegun at the National Security Council under President George W. Bush.

The stakes are significant. U.S. intelligence officials told Congress this month that it is unlikely that Kim Jong Un will agree to relinquish his nuclear arsenal, even as the regime has sought relief from punishing international economic sanctions. Trump, who initially demanded that North Korea give up its nuclear program quickly, has worked to lower public expectations since the Singapore summit. This week, Trump declared that he is in “no rush” as long as the Kim regime maintains a freeze on nuclear and ballistic missile testing that began in late 2017.

“Chairman Kim and I have a very good relationship. I wouldn’t be surprised to see something work out,” Trump told reporters at the White House on Wednesday.

In Biegun’s Stanford address, which came days before he traveled to Pyongyang for a meeting with Kim Hyok Chol, Biegun said he was well versed in the criticism that the Singapore agreement was threadbare. In meetings with former U.S. government negotiators and think tank experts in Washington, Biegun has sought advice over the potential pitfalls of negotiating with the North Koreans, including parsing their often blustery, occasionally opaque language.

Biegun acknowledged that there are “no new ideas” after nearly three decades of sporadic U.S. talks with Pyongyang, but he emphasized that his job was to identify openings for progress, said people who have met with him and spoke on the condition of anonymity to describe those conversations.

“We have big issues with North Korea on weapons of mass destruction, and that drove the sense of crisis . . . But in working with North Korea, we have a lot of other issues in the relationship that we have to resolve, too,” Biegun said at Stanford. “My theory of the case would be that we can resolve issues of disagreement outside of the weapons of mass destruction issue much more effectively through engagement than through the separation that we have right now.”

People who have met with Biegun described him as a politically savvy and seasoned negotiator who remains clear-eyed about his challenge and privately acknowledges the steep odds of a successful outcome.

Biegun was a well-known GOP aide during 14 years on the Hill, working as a foreign policy staffer for then-Sen. Jesse Helms (R-N.C.) and, later, as national security adviser for then-Senate Majority Leader Bill Frist (R-Tenn.). In between, he served on Bush’s NSC from 2001 to 2003 under then-national security adviser Condoleezza Rice.

Though Biegun’s foreign policy expertise and background is in Russia, one former colleague recalled Rice bringing him into North Korea-related meetings. In February 2002, when Bush visited the Korean demilitarized zone, his speechmakers were putting together remarks when Biegun lobbied to include the phrase, “Chairman Kim, tear down this wall!” — a pointed rebuke to then-North Korean dictator Kim Jong Il, the current leader’s father.

The Bush team, still dealing with the geopolitical fallout of the 43rd president’s “axis of evil” remark in his State of the Union address, nixed that language, said the former colleague who requested anonymity to discuss the private deliberations over the speech.

In 2008, Biegun joined the presidential campaign of Sen. John McCain (R-Ariz.) and drew a challenging assignment: briefing his vice-presidential running mate, then-Alaska Gov. Sarah Palin, on foreign policy.

“Whether it’s advising Sarah Palin or working on North Korea, it says something very favorable about Steve Biegun that he’s a guy very different people want in situations like this,” said Richard Fontaine, president of the Center for a New American Security, who worked on the McCain campaign with Biegun.

Biegun was working as the international government relations director at Ford when Pompeo tapped him for the role as special envoy to North Korea. The job came with significant risk. North Korea reneged on an agreement to suspend missile tests made during the six-party talks in the Bush era, scuttling years of painstaking negotiations with the United States, China, Japan, South Korea and Russia.

That collapse probably played a strong role in shaping the views of Bolton, who served in the State Department during the Bush administration. Ahead of the Singapore summit in June, Bolton publicly suggested that North Korea would be expected to pursue the “Libya model” of relinquishing its nuclear program wholesale — a prospect that angered Pyongyang, given that Libyan dictator Muammar Gaddafi was later overthrown and killed.

Bolton has continued to gripe internally about the talks and is said to have complained about Biegun’s approach directly to Pompeo, according to two people familiar with the issue who requested anonymity to discuss the internal discussions.

“A lot of critics say nothing works, everything fails, but that’s not true,” said Robert Carlin, a former U.S. intelligence analyst who was involved in North Korea talks during the Clinton administration and has met with Biegun. “A lot of people want to look way ahead, but there are stages and ways to reduce risk but not get to the endpoint right away.”

For Biegun, criticism comes with the territory, and it is ultimately muted by Trump’s support. On Christmas Eve, the president offered Biegun a vote of public confidence, tweeting a photo of them together in the Oval Office, along with Allison Hooker, an NSC staffer.

“Progress being made,” Trump wrote. “Looking forward to my next summit with Chairman Kim!”

Nakamura reported from Washington.