Former President Barack Obama has not supported the presidential candidacy of Kamala Harris because he does not believe she can beat Donald Trump, according to a source close to the Biden family.

After the Joe Biden shock resignation from the race on Sunday, and his immediate endorsement of the vice president, most of the Democratic elite have been quick to rally behind Harris, but Obama is a notable exception.

“Obama is very upset because he knows he can't win,” the Biden family source told The Post.

“Obama knows he's incompetent: the border czar who never visited the border, saying all migrants should have health insurance. He can't navigate the landmines ahead.”

“When you're running for president there are things you can and can't say.”

The 81-year-old Biden's turning point came after his disastrous performance against Trump in a televised debate in Atlanta last month that, The Post revealed it for the first timeit was part of an “elaborate setup” to remove him from the race.

But the source doesn't have high hopes for a televised debate between Trump and Harris. When Biden was still in the race, a second debate was scheduled for September 10.

“Wait until the debate… She can't debate. She's going to put her foot in her mouth about Israel, Palestine, Ukraine. She's going to say something really stupid,” the source said.

“Obama knew this was going to happen, Joe knew this was going to happen. Now he's going to have to answer real questions.”

The White House did not respond to The Post's request for comment for this story.

According to the source, Obama's hope was to get Biden out of the way and an article written by George Clooney in the New York Times asking him to step aside was part of that plan.

However, senior Democratic Party officials did not count on Biden to endorse Harris immediately, and “Obama was surprised” when Joe endorsed her, according to the source.

After Biden's impeachment, Obama, who did not return a request for comment Wednesday, wanted Arizona senator and former astronaut Mark Kelly “at the top of the ticket” when the Democratic National Convention is held next month, the source said.

Obama is “furious,” things haven't gone his way, and that's why he's not joining the Democratic Party's endorsement of Harris,” the Biden source added.

Another well-placed Democratic source said: “Obama being surprised by a smart policy decision makes sense when combined with his personal inability to see politics beyond his singular and immediate interests.

“Obama always thinks he's the smartest, coolest guy in the room. After all, he's friends with George Clooney.

Rod Blagojevich, the disgraced former Illinois governor who denounced Obama as a “threat to democracy” in a Wall Street Journal op-ed on Tuesday, he told The Post that he believes “Obama was instrumental” in getting Biden out of the race.

Blagojevich, a former Democrat who now supports Trump, was governor of Illinois when Obama became a state senator in 2005.

“This [Biden’s departure from the race] without him it wouldn't have happened [Obama],” Blagojevich told The Post.

“Without Obama, Biden remains the Democratic nominee. Obama was the conductor who convinced Biden to get on board.”

Obama ended his role as an Illinois senator when he was elected president in 2008, while Blagojevich was impeached in 2009 for trying to sell Obama's vacant Senate seat, impeached and convicted of corruption for one year after.

Obama has since persuaded Biden not to run for president in 2016 to allow Hillary Clinton a shot at the White House. among themselves, according to reports.

“It was not encouraging,” Biden later told the New York Times, referring to the former president, who believed at the time that the former secretary of state had a better chance of beating Trump.

“He's running the Democratic party like a Chicago chamberlain from the back shadows,” Blagojevich continued. “In this case it's a big back because it has a lot of big houses.