President Biden on Sunday faced his biggest challenge yet from inside the Democratic Party when four senior House leaders — including Rep. Jerry Nadler (D-NY) — reportedly told colleagues he should withdraw from the race.

The move by the Dem defectors dramatically ramps up the pressure on Biden, 81 — as they join five other House Democrats who have publicly called on the president to step aside because they fear he can’t beat GOP former President Donald Trump in November.

The new Biden skeptics are: Nadler, 77, the leading Democrat on the Judiciary Committee; Adam Smith (D-Wash.), ranking member of the Armed Services Committee; Mark Takano (D-Calif.), ranking member of the Veterans Affairs panel; and Joseph D. Morelle (D-NY), the highest-ranking Democrat on the Administration Committee, according to multiple reports.

They voiced their concerns during a private meeting of two dozen ranking House committee members, including Minority Leader Hakeem Jeffries (D-NY); Minority Whip Katherine Clark (D-Mass.) and Caucus Chair Pete Aguilar (D-Calif.). Jeffries, Clark and Aguilar listened to their House colleagues without weighing in, according to The Washington Post.

The disquiet stems from Democrats’ growing angst that Biden’s campaign will never recover after his disastrous debate performance June 27. In the Senate, Mark Warner (D-Va.) has already endeavored to rally his colleagues to nudge Biden to step aside, The Washington Post reported.

His spokesperson did not deny the report.

On Saturday, The New York Times reported that an anonymous senior White House official had also joined the chorus against Biden, saying the president has increasingly displayed more signs of age over recent months, appearing tired in public and speaking at a slower cadence.

Several big-name Democratic donors such as Abigail Disney have called on other financial party backers to close off their wallets until Biden drops out.

Other deep-pocketed donors have mused about diverting their resources from the presidential contest to down-ballot races in order to help stem any potential further hemorrhaging.

“I don’t think we can be polite and sugar-coat things. It will be brutal. Our base will not be energized. We are already having issues with young people. We are already losing different minority groups,” former Rep. Tim Ryan (D-Ohio) told CBS.

Alan Clendenin, Florida’s longest-serving member on the Democratic National Committee, said in a statement Sunday.

“The future of the Republic we know is in imminent danger and we must hold our country above our party or any one individual.

“I believe it is in the best interest of our country and the world that President Joe Biden step aside and allow Vice President Kamala Harris to carry forward his agenda as our Democratic nominee.”

The House of Representatives is set to reconvene Monday, and Democratic lawmakers will be peppered with questions from the press corps.

“The s–t is going to hit the fan on Monday when Congress returns,” a House Democrat told Axios. “People are scared about their own races. But they’re also worried about the country and about democracy.”

Biden has been adamant that he will stick it out unless the “Lord Almighty” intervenes and brushed aside a question about why some Democrats are outperforming him in critical states.

“That’s not unusual in some states. I carried an awful lotta Democrats last time I ran in 2020. Look, I remember them tellin’ me the same thing in 2020. ‘I can’t win. The polls show I can’t win,’ ” Biden told ABC News in an interview Friday, underscoring that he prevailed in the end.

Other Democrats have spoken out to express concerns about Biden’s fitness to see his re-election bid through — but stopped short of calling for him to bow out.

Perhaps the most pointed criticism came from California Rep. Adam Schiff, who told NBC News’ “Meet the Press” on Sunday that Biden “should be mopping the floor with Donald Trump. . . . It should not be even close.”

He added: “There’s only one reason it is close, and that’s the president’s age. ”