In the hours after last week’s presidential debate, a video clip began circulating that featured Jill Biden gingerly helping her husband step down from the event’s stage.

The snippet spread quickly on conservative media, along with an emerging narrative about President Biden’s weak performance: That the debate was a sweeping validation of the alarm bells they’d been ringing for years about his age.

Since well before he took office, pundits on the right have argued repeatedly that Mr. Biden, now 81, was too feeble for the job, playing up videos of him falling off a bike, tripping onstage or stumbling over words during speeches.

On the occasions when Mr. Biden has outperformed expectations — such as March’s State of the Union address — conservative critics have hinted that he must have been using a performance enhancing substance or, as former President Donald J. Trump bluntly put it in speeches leading up to last week’s debate, “jacked up” on drugs.

But after Mr. Biden’s performance Thursday, before an audience of 51 million people, concerns about the president’s fitness became widespread. For many media voices on the right, it appeared, it was hard not to gloat.

“The media woke up yesterday,” Brian Kilmeade, a Fox News host, said on Friday morning. “They couldn’t believe how bad Joe Biden did. Why are they just realizing it?”

He joined a chorus of conservative commentators excoriating the mainstream press, while vigorously patting their own backs, over coverage of the president. National Review, for example, ran a series of articles celebrating the fact that it had known Mr. Biden had a problem all along.

“We told you so, you fools,” read the headline of one particularly chiding story published Friday evening.

Indeed, many right-wing media voices have appeared so delighted with the feeling of vindication that they seem unsure of what to do next.

Some have chosen to push the story even further, turning in recent days to increasingly outlandish conspiracy theories about Mr. Biden and his campaign, framed by the notion that his quest for a second term in office wasn’t politics as usual, but a fraud on the American public and a sinister case of elder abuse perpetrated on an unwitting figurehead.

“It certainly fueled the fever swamp,” said Rob Stutzman, a Republican strategist. “No matter what happens, the conspiracists still think there’s some grand wizard that controls politics behind the scenes.”

In a series of posts on the social media site X on Monday, Tucker Carlson, the former Fox News host, claimed to have inside knowledge that Democratic elites had secretly conspired against Mr. Biden, alleging that a public statement of support by former President Barack Obama for the president was “disingenuous.”

The critic and novelist Walter Kirn, who hosted a debate watch party with the journalist Matt Taibbi viewed by 82,000 people, contended that the president was the target of a deep state “mob hit” calculated to embarrass him on national television and thus force him to quit the race against his own wishes.

Others, including the influential podcast host Ben Shapiro, pointed fingers at Ms. Biden, casting her as a power-mad latter day Lady Macbeth who is “insisting her dementia-ridden husband stay in the race.”

And the conservative talk radio host Mark Levin joined a chorus of critics comparing Ms. Biden unfavorably to President Woodrow Wilson’s wife, who secretly took over many executive duties after her husband suffered a debilitating stroke in 1919. “Jill Biden is the Edith Wilson of our generation,” Mr. Levin said.

Days after the debate, it was difficult to find conservative voices holding their fire — something Barrett Marson, a Republican campaign strategist, said they’d be well-advised to do given the very public crisis confronting Democrats.

“The debate doesn’t require hyperbole,” Mr. Marson said. “The best thing to do is just get out of the way and watch the meltdown.”

Still, there were a handful of conservative media voices willing to temper their glee, if only momentarily.

“I woke up feeling sad,” Greta Van Susteren, a host on Newsmax, posted on X on Friday morning. “I did not enjoy watching an elderly man struggle for 90 minutes with the world watching.”