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.

Though most Brits now regret leaving the European Union, the anti-establishment mood still pervades the Western world. France, Britain and the United States have been shaken by the pandemic, inflation, Russia’s full-scale 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 conducted last week showed that 72 percent of registered voters said 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 thought this last week about Mr. Trump. A Morning Consult survey after the debate showed 60 percent of voters thought 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 persuade 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 it’s not as easy as he thinks. Instead of grappling with this hard truth, Mr. Biden’s family reportedly blames staff. Loyalists disparage as dismalist-catastrophists anyone who hasn’t rallied behind the president.

On Tuesday, Rep. Lloyd Doggett (Tex.) became the first sitting Democratic member of Congress to call for Mr. Biden’s exit from the race — publicly saying what many continue to express privately. Yet, the president is keeping a light schedule this week. He declined to answer questions after delivering brief remarks on Monday night. ABC News announced that Mr. Biden will give his first interview since the debate to George Stephanopoulos on Friday, eight days after his halting performance. The White House announced Mr. Biden will hold a news conference during the NATO summit. That’s not until next week.

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.