The loudest public alarm about Joe Biden ’s mental acuity came in February with the release of special counsel Robert Hur’s report, a document he produced after spending five hours interviewing the president that revealed Biden displayed significant memory problems.

Democrats worried. Then most of them followed the president’s lead and dismissed it as partisan hackery. Biden’s closest advisers defiantly beat back suggestions that the 81-year-old president showed signs of decline .

Minutes into Thursday night’s presidential debate , the concerns began gushing into the open.

Yet they had already become increasingly apparent in Washington’s corridors of power and across the world for months. In interviews, top officials abroad and Democrats said they have witnessed other moments when Biden’s behavior concerned them. Some were quickly relieved when Biden appeared to regain his footing. Others were left shaken by the experiences.

European officials had already been expressing worries in private about Biden’s focus and stamina before Thursday’s debate , with some senior diplomats saying they had tracked a noticeable deterioration in the president’s faculties in meetings since last summer. There were real doubts about how Biden could successfully manage a second term, but one senior European diplomat said U.S. administration officials in private discussions denied there was any problem.

The White House disputed the characterization that Biden has stumbled on the world stage. “Foreign leaders see Joe Biden up close and personal,” said National Security Council spokeswoman Adrienne Watson. “They know who they are dealing with, how effective he has been, and how important his leadership is on the world stage.”

Diplomats described Biden’s performance at the Group of Seven summit in Italy in mid-June as mixed, with Biden appearing physically frailer than in the past but alert in many of the most important discussions.

German Chancellor Olaf Scholz receives birthday wishes from European Council President Charles Michel, U.S. President Joe Biden, Italian Prime Minister Georgia Meloni, French President Emmanuel Macron, Canadian Prime Minister Justin Trudeau, British Prime Minister Rishi Sunak and European Commission President Ursula von der Leyen on the second day of the G7 summit, in Savelletri, Italy, June 14, 2024. Steffen Kugler/BPA/Handout via REUTERS

Biden missed the summit’s dinner party in a medieval castle, an off-camera and less- scripted part of the summit in which leaders often exchange views more candidly. He was the only G-7 leader not to attend the meal; the White House told reporters in advance he wouldn’t be there because it would be a “jam-packed two days” of meetings. Biden instead held an event with Ukrainian President Volodymyr Zelensky and had a news conference.

The next day, Biden followed through on his plans to forgo a Swiss peace summit on Ukraine to attend a California fundraiser instead, frustrating Ukrainian officials who had organized the conference in Switzerland directly after the G-7 in the hope that Biden would come from Italy.

Top White House officials disputed any portrait of a president slipping during his European trip, which included 12-hour days chock-full of meetings. “He didn’t miss a beat,” said Daleep Singh, the deputy national security adviser for international economics who participated in meetings with Biden. Singh said that there has been no change to the “forceful and substantive rigor” that Biden brings to meetings with foreign leaders.

Officials said that Biden’s performance and focus can vary significantly between meetings and even within a meeting. Two senior European officials cited a European Union-U.S. summit in October in Washington at which Biden struggled to follow the discussions. Both said he stumbled over his talking points at several moments, requiring Secretary of State Antony Blinken to intervene and point out the lines he should use.

“I saw none of that,” said Mark Gitenstein, the U.S. ambassador to the European Union, who attended the meeting. He said Blinken intervened just to help Biden find a tab for a document in a large notebook. He also said that during the meeting Biden was discussing the release of hostages held by Hamas, which drew his attention for part of the meeting.

“He was whispering to them,” Gitenstein said, referring to aides. “And he got an answer and was pleased with the results. He then turned around and announced that hostages were released. That was the main thing that happened.”

Gitenstein added that the meeting was large and included people who haven’t spent time with Biden and therefore don’t know him well.

Worries in Europe

“The reading in Europe is that this has been an unmitigated disaster,” said Nathalie Tocci , director of the Institute of International Affairs in Rome and a former adviser to the EU’s foreign-affairs chiefs, referring to Biden’s attempts to reassure voters worried about his age. European officials and prominent commentators, she added, “have been talking about it. It’s something that has been known, always, that his age is his main Achilles’ heel.”

In the hours after Thursday’s debate, Polish Prime Minister Donald Tusk told reporters that the Democrats have a problem.

“I was afraid of this. It was to be expected that in a direct confrontation, in a debate, it would not be easy for the president,” said Tusk, who has known Biden for years. Asked what he thought of proposals to replace Biden with another candidate, he said: “They definitely have a problem. The reactions have been unambiguous.”

Norbert Röttgen, a senior lawmaker in Germany’s center-right main opposition party and the former chair of its parliament’s foreign-affairs committee, said on X: “The Democrats must switch horses now.” Slovenian former Prime Minister Janez Janša said he bet months ago that Biden would no longer be on the ballot in November.

When Biden was in France to commemorate the 80th anniversary of D-Day in June, he struggled. During a bilateral meeting in Paris with Zelensky, Biden spoke so softly that reporters brought in to document the meeting between the two men couldn’t initially hear the American president. Zelensky, a non-native English speaker, could be heard clearly.

During the brief exchange, Biden flubbed when explaining what the Ukrainians would do with a new tranche of $225 million in funds that the U.S. was sending. Biden told Zelensky that money was “to help you reconstruct the electric grid.” Officials traveling with the president explained that the money was actually a series of munitions including air defenses that could protect the electric grid, among other targets, and not for rebuilding the electric grid.

Back in Washington for a brief time between his trips to France and Italy, Biden appeared at a Juneteenth concert at the South Lawn. Video footage of the event showed him clapping noticeably out of time to the music. He needed assistance from the person seated next to him when it was time for him to get up.

White House press secretary Karine Jean-Pierre , when asked about his awkward behavior at the event, dismissed the question. “That is just not a health issue,” she said.

Throughout the ups and downs, first lady Jill Biden has been a strong supporter of her husband’s decision to run and stay in the race, according to two people familiar with the situation. Biden’s top aides insist publicly and around various Washington dining room tables that the president is mentally sharp. As recently as May, one aide in frequent contact with the president insisted that there has been absolutely no change in his performance.

And, when confronted with evidence that Biden is deteriorating, the White House pushes back hard, including when The Wall Street Journal reported this month that Biden showed signs of slipping in negotiations with congressional leaders. In February, before the Hur report was released on the probe into Biden’s handling of classified documents, White House lawyers sought to strip references about Biden’s memory from the report. The White House team of communications officials labeled Hur’s comments on Biden’s memory as “gratuitous” as they expressed relief that he wasn’t recommending criminal charges against Biden.

Donor frustration

Contributors have for months privately raised concerns about Biden’s lack of spontaneity in settings with his biggest supporters.

At a September 2023 fundraiser in New York, Biden retold the same anecdote twice—leaving at least one attendee shaken and worried about his age. In February, Biden used a teleprompter at a fundraiser in Los Angeles, and questions were screened in advance, according to a person familiar with the matter—frustrating some donors who had expected a more free-flowing exchange.

During a May fundraiser in Washington state, Biden appeared to lose his train of thought as he talked about Israel and at one point paused for five seconds before continuing, according to a pool report from a journalist who attended. Biden said: “The cease-fire would begin tomorrow. It all has to do…you know, we’ve not…anyway, I guess I shouldn’t get into all this about Israel but…”

Some major Democratic donors have sat this race out, believing that the ticket might change or that Biden is too old, according to two people familiar with Biden’s fundraising operation. So far the money has come in at such a fast clip that those naysayers have been ignored.

Write to Laurence Norman at laurence.norman@wsj.com and Drew Hinshaw at drew.hinshaw@wsj.com