Opinion | Biden needs to be clear-eyed about the election. France and the U.K. show why.

Across the English Channel, Prime Minister Rishi Sunak had until next year to call elections in Britain but picked July 4. Now he looks certain to be ousted on Thursday. Polls show the Conservative Party might wind up with the fewest seats since its founding in 1834. After 14 years in power, and five prime ministers in eight years, the Tories have worn out their welcome, and they’ve run a sclerotic campaign that has only underscored internal divisions.

On this side of the Atlantic, Mr. Biden demanded the earliest-ever general election presidential debate. His team imagined a face-to-face clash with Donald Trump would give him an opportunity to assuage fears about his age and stamina while drawing attention to the binary choice before voters. The president’s advisers got the accommodations they demanded, including muted microphones and no studio audience. Instead, despite a week of preparation and rest, Mr. Biden’s performance ignited clamoring for him to end his campaign.

Paris, London and Washington have distinct political cultures and institutions, and each of these races is different. But they’re nevertheless affected by a shared geopolitical weather system. The domestic politics of these three nuclear powers are more closely intertwined than they may appear. That’s why the Brexit referendum in June 2016 foreshadowed Mr. Trump’s victory five months later.

While most Brits now regret leaving the European Union, the anti-establishment mood still pervades the Western world in 2024. All three countries have been shaken by the pandemic, inflation, Russia’s invasion of Ukraine and strong anti-immigration sentiment. That’s why Labour leader Keir Starmer, poised to be Britain’s next prime minister, has attacked Mr. Sunak from the right on this issue, calling him “the most liberal prime minister we’ve ever had on immigration.”

Considering these headwinds, Mr. Biden’s resilience and durability in the polls until now are remarkable. It’s a consequence of Mr. Trump being so polarizing, combined with the strength of the U.S. economy and, specifically, its job market. But the president’s debate performance has brought his limitations to the fore.

A CBS News-YouGov poll published Sunday shows that 72 percent of registered voters say Mr. Biden “does not have the mental and cognitive health to serve as president.” It was 65 percent a few weeks ago. For comparison, 49 percent of respondents think this about Mr. Trump. A Morning Consult survey after the debate shows 60 percent of voters think Mr. Biden should be replaced as the Democratic nominee, including 47 percent of Democrats and 59 percent of independents.

To win, Mr. Biden would need to convince millions of people who do not think he’s up to the presidency to cast ballots for him. What’s happening in France and Britain shows that it’s not as easy as he thinks. Instead of grappling with this hard truth, Mr. Biden’s family reportedly blames staff.

The president’s reelection campaign insists that he’s not going anywhere. To date, no prominent elected Democrat has publicly called on Mr. Biden to step aside — only commentators. Loyalists disparage as dismalist-catastrophists anyone who hasn’t rallied behind the president. Rather than make that case for Mr. Biden, the Democratic National Committee is reportedly thinking about rushing through his formal nomination in mid-July to stamp out talk of replacing him.

Mr. Biden should be giving interviews and holding news conferences. Yet his schedule this week is light, which only bolsters reports that he functions best from 10 a.m. to 4 p.m. in a job that requires around-the-clock attention. If Mr. Biden stays in the race and Republicans win the White House and both chambers of Congress in four months, Democrats will regret that they didn’t think more carefully during this moment when they might still have been able to avert such a lamentable outcome.