Top Democratic donors threaten to pull the plug on funding if Biden doesn’t resign, while furious party members reveal their “worst fears” about the ailing president have now been confirmed: “How stupid do they think we are?”

The growing Democratic outrage over President Joe Biden’s shockingly poor performance in Thursday night’s debate appears to be turning into a full-blown partisan revolt.

“For the sake of Biden and the country, he should step aside immediately,” Whitney Tilson, a major Democratic donor and former hedge fund manager, told DailyMail.com. “The fact that it’s now three days later and Biden has done nothing to reassure us confirms my worst fears.”

Tilson has donated more than $300,000 to the party in recent years.

Days after the President struggled to maintain his train of thought and trailed off mid-answer into incomprehensible gibberish during the CNN debate, left-wing do-gooders are still outraged.

On Saturday, Biden appeared at a fundraiser in New York (in a glitzy Long Island seaside town) and, according to an aide, read a straightforward 15-minute speech from a teleprompter, before leaving without taking questions.

“Doing that for the next five months is not going to be enough,” the fundraiser guest said, stressing that Biden’s understandably anxious supporters deserve assurances that he is capable of defeating Donald Trump in November.

But later Saturday night, Democratic National Committee Chairman Jaime Harrison and Biden campaign manager Julie Chavez Rodriguez only further fueled concerns on a conference call with undecided DNC members.

“The call was akin to a rousing speech,” said John Verdejo, a North Carolina party member who told the Mail he was surprised when the call’s organizers didn’t address the “elephant in the room” — Biden’s inability to speak clearly.

Other DNC members went even further than Verdejo in their criticism. Some said they felt “duped” by campaign officials who were on the call and who seemed to deny there was anything to worry about. The official line from the Biden team appears to be: There’s nothing to see here.

“By every metric that matters, the data shows that (the debate) did nothing to change the perception of the American people,” insisted Jen O’Malley Dillon, chair of the Biden-Harris 2024 campaign, in a memo published this weekend.

Almost no one believes that story.

“The data!?” tweeted Mike LaRosa, former press secretary to first lady Jill Biden. “If there is data that contradicts the impact of what we all saw, WHERE THE HELL IS IT?”

Democratic donor Whitney Tilson also minced no words.

“They’re pissing on our legs and telling us it’s raining. It’s insulting. How stupid do they think we are?”

In fact, a CBS News/YouGov poll conducted during the two days following the CNN debate found that 72 percent of registered voters do not believe Biden has the “mental and cognitive health necessary to serve as president.” That figure is up from 65 percent in early June.

On Monday morning, Biden was still holed up at Camp David with his family, who had gathered for a post-debate photo shoot with famed photographer Annie Leibovitz.

He did not return to the White House until Monday evening, when he made brief remarks on the Supreme Court’s ruling on presidential immunity before leaving the podium without taking questions.

A reporter was heard shouting: “Are you going to quit?”

This apparent lack of urgency on his part is already raising alarm among Democratic lawmakers, who fear that Biden’s missteps could end up damaging his own electoral prospects.

“Everyone is waiting to see what the president is going to do and say,” said one House member. “I think the president himself should make a significant effort to reach out to congressional leaders.”

An aide to a House Democrat who faces a competitive Republican challenger in November echoed those concerns, telling the Mail there are real worries that dissatisfaction with Biden will depress overall voter turnout.

According to Brett Bruen, a former Obama White House adviser, this growing Democratic panic is entirely predictable.

“Donors are the canaries in the political coal mine,” he said. “As the polls over the last day have indicated, there are repercussions from Biden’s performance on all candidates.”

“The idea that everything will get worse if Biden steps aside ignores the fact that it’s already a huge mess,” he added.

Perhaps the biggest challenge facing the party is that even if the president were to step down, there would be no obvious candidate to replace him and no clear path forward.

Tilson’s prescription for Biden, then, is to ditch the notes, teleprompters and pre-screened questions, and prove he’s up to the job by holding weekly press conferences or sitting down with the New York Times editorial board to persuade them that it was wrong to ask him to drop out of the race.

“It’s an absolutely sensible strategy,” said Bruen, a former Obama adviser. That is, he added, provided Biden doesn’t suffer another serious public blunder.

“There is a great deal of anxiety among Biden’s advisers that one more high-level moment could increase the already significant pressure among Democrats for the president to step down,” he said.

Not to mention the unprecedented challenge of switching to a major party’s likely nominee seven weeks before a nominating convention and four months before a general election.

The most discouraging news for those who want Biden to withdraw came from Chavez Rodriguez on Saturday, who said that if Biden withdraws, only Vice President Kamala Harris will be able to use the funds raised by his campaign.

The $91.2 million raised by the Biden-Harris ticket through May 31 is not transferable to any other candidate, according to Federal Election Commission rules. So the party may find itself with its hands tied.

A distressing thought, indeed, for party officials who know full well that Harris consistently polls as the most unpopular vice president in history.

As one senior official observed in exasperation: “The irony is that anyone but Kamala Harris would have more of a chance of beating Donald Trump.”