Impeachment is a “mortal threat” to Donald Trump’s presidency and the White House needs to start taking it more seriously, former chief strategist Steve Bannon told The Post.

“This is serious. As sure as the turning of the earth, he is going to be impeached by Pelosi in the next six weeks,” Bannon said during a wide-ranging interview with The Post. “Nancy Pelosi is very focused.”

And if Trump makes it out of a House impeachment probe alive, the 65-year-old ex-Breitbart News chairman says the president may have to face surprising — and viable — challengers in 2020 in Mike Bloomberg or Hillary Clinton.

Bannon said he’s been dismayed by what seems to be an uncoordinated White House approach to impeachment.

“I don’t feel comfortable when I see the chief of staff, Mick Mulvaney,” Bannon said, citing a disastrous press conference Thursday in which Mulvaney confirmed Trump’s decision to withhold military aid to Ukraine was part of a quid pro quo. He later walked that back.

“The problem we have is that the president needs a team around him and somebody has got to step up and make a play. Trump can’t do everything,” Bannon said. “There is just no coordination with the team.

“The fake news and witch hunt stuff is not working.”

The former 2016 campaign boss said Trump shouldn’t look to dump off-message surrogates like Mulvaney or Rudy Giuliani, but rather bring in people to share the burden.

Bannon advised the White House to establish an impeachment squad.

“You need to augment the legal team,” he said, adding that what worked on the probe by Robert Mueller into Russian interference “was bifurcation of the White House Counsel’s office. You need … a team put together than can focus on [impeachment] 24/7.”

And while the GOP-controlled Senate would likely never find Trump guilty in an impeachment trial, Bannon repeatedly refused to offer a full vote of confidence to leader Mitch McConnell — who he called an adversary — and instead expressed frustration that the Senate leader might opt for a drawn-out public trial.

Bannon remained bullish overall about Trump’s chances in 2020, dismissing the current crop of Democratic candidates.

“Nobody on that stage can beat Trump at least as far as right now. The only person that could beat Trump that is currently in the field is Trump,” he said.

He predicted a total Biden implosion by the end of the year and a centrist revolt against Elizabeth Warren.

If Biden falters and Warren appears to be marching toward nomination, “Bloomberg and Clinton, both will … get into the race,” Bannon said. “Nobody is on the stage. The Cory Bookers and Kamala Harrises haven’t gotten enough traction to compete with Warren. The hedge fund investment banking corporatist community of the Democratic Party is not prepared to have them.”

Clinton and Bloomberg would be much stronger than the current field, Bannon conceded.

“Bloomberg or Clinton could be very competitive,” he said. “She is a very formidable candidate and I think Bloomberg would be very formidable.”

He said Democrats looking for an edge should do their best to force Team Trump to spend resources in his “southern arc” — potential swing states like Arizona, Texas, Georgia and the Carolinas.

Whoever ended up as the Democratic nominee would also “have to live in Pennsylvania, Michigan, Wisconsin,” he said. In her 2016 race, Clinton famously neglected these states, and they turned against the party for the first time in a generation.

Since leaving the White House in 2017, Bannon has traveled the world at a breakneck pace, speaking at conferences, meeting with billionaires and promoting his vision of populist nationalism wherever he can find a friendly audience.

He has also taken a growing interest in China, which he views as the primary threat to the United States. Bannon spoke with The Post shortly after a screening of his new film, “Claws of the Red Dragon” in New York. The movie is a loose retelling of the arrest of Huawei CFO Meng Wanzhou in Canada last year.