Then-Senate Majority Leader Chuck Schumer warned President Biden over the summer that he’d be remembered as “one of the darkest figures” in American history if he allowed Donald Trump and Republicans to win the November elections — and broke down in tears after urging his party’s presumptive nominee to drop out of the 2024 contest.

Schumer, now Senate minority leader after the GOP regained control of the upper chamber, relayed new details about a high-stakes meeting he had with Biden, now 82, just eight days before the president abandoned his re-election effort, according to an adapted excerpt from a forthcoming book published Friday in the New York Times.

“If you run and you lose to Trump, and we lose the Senate, and we don’t get back the House, that 50 years of amazing, beautiful work goes out the window,” the Brooklyn Democrat told the oldest-ever president at Rehoboth Beach, Del. July 13. “But worse — you go down in American history as one of the darkest figures.”

“If I were you,” Schumer added, “I wouldn’t run, and I’m urging you not to run.”

Biden reportedly listened intently and at times expressed surprise — but didn’t argue — as Schumer shared shocking details about how little confidence Democrats had that he could defeat Trump, 78, a second time in November.

Schumer, now 74, said he had a whip count showing just five members of the upper chamber wanted Biden to stay in the race after his disastrous June 27 debate showing against Trump.

“My guess is you have about a 5% chance,” the New Yorker said, adding: “None of your pollsters disagree with me.”

Biden, who was seemingly unaware of how poorly he was faring, responded that he needed at least a week to consider his position — an exchange The Post reported the day after the president announced the end of his campaign July 21.

“You’ve got bigger balls than anyone I’ve ever met,” the president told Schumer gamely as the two parted, placing both his hands on the senator’s shoulders.

The two then hugged, and Schumer walked back to his car — where he began to weep.

Fighting back tears, he told aides that he was unsure whether the president would take his advice, but felt he had made a strong enough case to alter the course of the 2024 contest.

The tell-all about the Senate Democratic leader’s behind-the-scenes effort to encourage Biden’s near-unprecedented exit in the middle of an election year is part of “Mad House: How Donald Trump, MAGA Mean Girls, a Former Used Car Salesman, a Florida Nepo Baby, and a Man With Rats in His Walls Broke Congress,” written by Times reporters Annie Karni and Luke Broadwater.

The book recounts how former President Barack Obama orchestrated the Schumer meeting after Biden refused to budge despite public denunciations from several Democrats and a private meeting with party stalwarts on Capitol Hill.

At that meeting, Sen. Jack Reed (D-RI) pressed Biden-Harris campaign advisers to submit the president to two neurological tests — and publish the results — while Sen. Sheldon Whitehouse (D-RI) warned that further delaying public comment on Biden’s mental state would amount to “lying.”

According to Karni and Broadwater, Biden was audibly “shouting” as Schumer entered the screened-in porch of the Delaware beach house, with the president defending his viability at the top of the ticket on a Zoom call with other, highly skeptical Democrats.

The authors note that by this point in Biden’s term, Schumer had noticed during phone conversations that the president would express confusion about why he had called the Senate Democratic leader in the first place, but had initially shrugged them off.

The Schumer-Biden meeting was overshadowed by an assassination attempt against Trump on the same day at a campaign rally in Butler, Pa., pushing speculation about Biden’s political future briefly to the back burner. 

Schumer, who recently sidestepped questions about whether he and other influential Democrats had misled voters about Biden’s mental fitness, had privately been telling party donors to keep quiet about their own concerns following the debate flop.

“Do not be public,” he told allies last July. “That will get his back up, and you’ve got to let the dust settle. But if you can, call whoever you know in the campaign. Call the White House.”

The book excerpt also revealed that House Minority Leader Hakeem Jeffries (D-NY) helped pave the way for Vice President Kamala Harris‘ brief and unsuccessful bid for the White House. 

“The most powerful narrative in American politics is change. Vice President Harris would represent change,” Jeffries told Biden adviser Steve Ricchetti and White House chief of staff Jeff Zients, trying to pave the way for a coronation of Harris atop the Democratic ticket.

Stubborn Biden advisers prolonged the palace coup for more than three weeks, with Ricchetti insisting to Jeffries: “We can get through it. We think our allies on the Hill are wrong.”

Since then, multiple reports have surfaced confirming the president’s cognitive decline was evident — and hidden by those closest to Biden — from the first months of his term.

For his part, Biden has also maintained that he would have triumphed over the Republican nominee had he remained in the race.

“I think I would have beaten Trump, could have beaten Trump, and I think that Kamala could have beaten Trump, would have beaten Trump,” he told reporters earlier this month.

Harris, 60, lost all seven critical swing states as Trump carried both the Electoral College and the popular vote to win back the White House on Nov. 5.

“Mad House,” from Penguin Random House, is due out March 25.