During Thursday night’s debate, President Biden told former President Donald J. Trump that the United States is the “envy of the world.”

After watching their performance, many of America’s friends in Asia beg to differ.

In Seoul, Singapore, Sydney and beyond, the back-and-forth between the blustering Mr. Trump and the halting Mr. Biden set analysts fretting — and not just about who might win.

“That whole thing was an unmitigated disaster,” wrote Simon Canning, a communications manager in Australia, on X. “A total shambles, from both the candidates and the moderators. America is in very, very deep trouble.”

Countries that have hoped the United States could balance a rising China and deter North Korea’s nuclear ambitions spent the past four years trying to rebuild ties after Mr. Trump’s first term deeply rattled alliances in the region. The debate on Thursday night immediately resurfaced serious questions about how U.S. politics might affect stability across Asia.

Chan Heng Chee, who served as Singapore’s ambassador to the United States from 1996 to 2012, said the quality of the debates has deteriorated compared with previous ones. Mr. Biden’s disjointed performance and Mr. Trump’s repeated attacks and factual inaccuracies unsettled those who rely on the U.S. to act as a trusted global partner.

“Now everyone is watching for visuals,” Ms. Chan said. “Do the candidates look like they are able to do the job, or is age a problem? Facts do not matter now, and civility has totally gone out of the window.”

In Japan and South Korea, analysts detected a shift in the political winds toward Mr. Trump, and it prompted renewed questions about Mr. Biden’s age and ability to project strength.

“It was clearly a Trump win and a nail in the coffin for the Biden campaign,” said Lee Byong-chul, a professor at the Institute for Far Eastern Studies at Kyungnam University in Seoul. “Trump looked healthy compared with Biden, who came across as an old, stammering hard-to-hear grandfather. We must now brace ourselves for a second Trump administration.”

In Japan, America’s largest ally in Asia, officials have almost always been assiduous about declaring that they are happy working with whomever the United States elects. But Mr. Trump’s comments during the debate about how he does not want to spend money on allies are likely to revive anxieties about how his approach to international relationships is transactional rather than enduring.

“My guess is that the Japanese policymakers are thinking, ‘OK, it’s going to be Trump quite likely, so we have to cement institutional ties as much as possible so he can’t undo them,’” said Koichi Nakano, a political scientist at Sophia University in Tokyo. “That is like tying yourself to a mast that may be sinking very soon, so it’s a false illusion of security.”

If Trump wins, though, Japanese officials may feel less stress about the demands he might make on Tokyo to pay more for its own defense or for the basing of American troops in Japan.

In the past two years, Japan has vowed to increase its defense budget and stretched the limits on what it could do under its pacifist Constitution, including purchasing more fighter jets and Tomahawk missiles, measures that Mr. Trump pushed during his visits to Japan as president.

The increased spending and military purchases “are in line with what he has been thinking,” said Ichiro Fujisaki, a former Japanese ambassador to Washington, referring to Mr. Trump. “If we are going in the opposite direction of what he was saying,” Mr. Fujisaki said, “we would have to review our position, but we’re not.”

Across the region, one of the most pressing concerns is how Mr. Trump might exacerbate widening tensions with China or undermine the fragile stability in the region.

If Mr. Trump wins, Washington would likely pursue a strategy that seeks to elevate U.S. influence in the Indo-Pacific in opposition to China, “but in a way that would prioritize U.S. pre-eminence, and not necessarily the network of alliances and partnerships it has in a collective sense,” said Don McLain Gill, a Manila-based international studies lecturer at De La Salle University. “Being a transactional leader, there are concerns that Trump may abandon key areas of U.S. commitment, such as Taiwan.”

On social media in China, the presidential debate was a top trending topic on the platform Weibo. Official Chinese media outlets largely played it straight, reporting each candidates’ remarks — and their lack of a handshake — without adding much commentary.

But in comments online, some users compared Mr. Trump’s red tie to a Communist red scarf, and some social media commentators jokingly called Trump “nation builder” because of how his leadership could accelerate China’s global rise.

The stock of Wisesoft Co., a Chinese company whose name in Mandarin translates to “Trump Wins Big,” jumped 10 percent in trading on Friday in Shenzhen, according to Bloomberg.

Social media merriment aside, Shen Dingli, a Shanghai-based international relations scholar, said the debate had only reinforced something the Chinese government has long thought: No matter who the next president is, U.S. policy toward China is only likely to harden, if not the same.

The candidates sparred over who had done a better job managing trade with China, Mr. Shen said, when in reality the Biden administration had continued Trump-era tariffs.

“Even if the Democrats urgently choose a new, younger candidate, they will all be set on treating China as a long-term, strategic threat, even more so than Russia,” he said. “I believe Chinese leaders don’t have any illusions.”

What was clear after Thursday’s debate was that few in the region feel optimistic about any of the electoral options in the United States.

Even in India, which clings to its political neutrality, analysts were openly alarmed by the sight of the two men vying to be the leader of the free world trying to prove their physical mettle by comparing golf handicaps.

“It’s very dismaying, because in India, we are usually accustomed to look up to the U.S. — it’s a powerful nation, they decide the fate of the world,” said Tara Kartha, a former senior official in India’s national security council. “And this country, where the two leaders are talking about golf in terms of fitness, and in terms of their ability at all, it’s very, very frightening.”

With India having worked to overcome a history of mistrust of the United States in order to combat a shared threat from China, the current uncertainty, particularly if Mr. Trump returns to power, weighs heavily on a country that has a hot border dispute with its neighbor.

“We are now hedging with China; we are not going beyond a point precisely because of this,” Dr. Kartha said. “Because you are not really sure what’s going to happen to the U.S.”

Kasit Piromya, Thailand’s former foreign minister from 2008 to 2011 and a former ambassador to the United States, lamented the state of American politics.

“Where are the good ones? Where are the brave ones?” Mr. Kasit said, adding that it was now incumbent on countries in Southeast Asia to have a foreign policy vision of their own. “Why should I wait for Trump to be bad? I should be able to organize myself and maybe work with other friends.”

Ja-Ian Chong, an associate professor of political science at the National University of Singapore, said President Biden looked very tired, while former President Trump sounded more unpredictable in terms of what he would expect from other friendly countries and how he would deal with China.

“It creates new problems, for trying to manage the relationship with the U.S.,” he said. “In general, policymakers want a clear, committed and steady U.S. presence. One that is wavering, weak and uncommitted is as troubling as one that is mercurial and inconsistent.”

“You’re looking at the two extremes,” Mr. Chong added. “It’s hard to imagine right now what a more moderate center for the United States looks like.”

Reporting was contributed by Damien Cave, Sui-Lee Wee, Choe Sang-Hun, Vivian Wang and Mujib Mashal.