SEOUL, South Korea — Japanese Prime Minister Fumio Kishida arrived in Seoul Friday for a two-day summit with South Korean President Yoon Suk Yeol, just weeks before exiting the premiership.

Buffeted by dangerously low popularity ratings, Mr. Kishida decided in August not to run in the leadership contest of his political machine, the ruling Liberal Democratic Party.

The LDP will hold an internal election for its presidency on Sept. 27, with the winner automatically becoming the nation’s prime minister. Assuming no further step-downs, the new party boss will lead the LDP into battle in a general election that must be held by July 2025 at the latest.

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The Seoul summit offers Mr. Kishida the chance to leave office on a high note, provide support to Mr. Yoon and send messages to his successor.

Anticipated agenda items in Seoul include both people-to-people and security policy upgrades.

Post-COVID, bilateral business and touristic travel has soared, and new visa regulations easing entries to both countries are anticipated. The Asian neighbors are also expected to formalize an agreement allowing for the evacuations, by each other’s security forces, of each others’ nationals, from geopolitical hotspots.  

However, there may be larger, unspoken agendas, experts say.

One is for the two Asian democracies to compare notes on the November presidential election in the United States.

Another is to deepen nascent but fragile security ties that Mr. Kishida and Mr. Yoon have forged between Seoul and Tokyo, to the point where future leaders cannot unravel them.

What is in it for whom?

The summit promises an upbeat sayonara for Mr. Kishida.

“Kishida needs to fortify his legacy: He is really unpopular but is very confident and proud of what he did,” said Lim Eun-jung, an expert on Korea’s regional relations at Kongju National University. “He wants to send some messages to his successor — whoever that may be.”

Mr. Yoon told a press conference last week that he expected to work closely with Mr. Kishida’s successor. Next year holds the potential for upbeat Korea-Japan vibes: 2025 marks the 60th anniversary of the establishment of diplomatic relations.

For Mr. Yoon, who is suffering from his own poll woes, the meeting offers a chance to further entrench his flagship foreign policy initiative: upgrading Seoul-Tokyo ties.

Defying the feelings of vocal sections of the domestic polity, media and civil society, Mr. Yoon, who took office in 2022, made improving long-troubled relations with Japan the centerpiece of his overseas policy.

It has been a high-risk move — but one that Mr. Yoon has stuck to. He has downplayed historical frictions over remuneration for wartime forced labor, and also over disputed signage related to Korean victims at Japanese historical sites.

In contrast, the prior Seoul administration closely linked early 20th century history to 21st century politics.

Mr. Kishida and Mr. Yoon have met 12 times — a major increase in meetings over those of their respective successors. During their terms, they also took part in a first-ever trilateral summit with U.S. President Biden.

Though both capitals have separate defense treaties with Washington, no formal alliance binds the three. The Kishida-Yoon’s working relationship has enabled emergent trilateral security ties.

“Kishida wants to help Yoon, and his successor, and even send messages to Washington and to any other countries that are watching,” Ms. Lim said.  

The American angle

Healthy Seoul-Tokyo ties have been well received by security pundits in Washington, where hopes for a Northeast Asian trilateral military partnership to counter China, North Korean and Russia have long been stymied by historical grievances.

Regarding the future inhabitant of the Oval Office, “The choices are stark,” said Haruko Satoh, who teaches Japan’s relations with the region at the Osaka School of International Public Policy. “Tokyo and Seoul would want to compare notes.”

The Biden-led Democratic administration has focused on forging alliances among Indo-Pacific democracies. That does not appear to be a priority among the Donald Trump-led Republicans.

With U.S. support, Seoul and Tokyo have expanded sharing of general intelligence and North Korea missile-related analyses. This July, with the United States, they inaugurated a new series of annual, trilateral naval exercises, “Freedom Edge.”

However, one plan, announced at a May meeting in Virginia between the deputy foreign ministers of the three countries, has not come to fruition.

“We are committed to forming some sort of coordinating body, a secretariat of some kind,” U.S. Under Secretary of State Kurt Campbell said at the time. “We have been tasked by our leaders to support that, to make sure that we maintain forward momentum on the important work of this trilateral grouping.”

Actors in both Japan and Korea — respectively, hardcore nationalist right-wingers, some of whom hold deep-rooted anti-Korean prejudices, and dyed-in-the-wool anti-Japanese left-wingers, focused on keeping historical animosities bubbling — might like current amities nipped in the bud.

However, the wider geopolitical environment, which couples Chinese expansionism with tightening North Korea-Russian ties, may force them to bow to pragmatism.

“Both sides have to move on,” argued Ms. Satoh, who expects the next LDP leader to continue Mr. Kishida’s regional security policies. “There are bigger fish making bigger splashes and Japan and Korea can’t afford to be held back by those voices.”