Former Vice President Kamala Harris' campaign reportedly cut off any campaigning with former President Joe Biden after he allowed himself to be photographed wearing a hat supporting President Donald Trump ahead of the 2024 election, according to an upcoming book by CNN's Jake Tapper and Axios' Alex Thompson.

The incident took place on September 11 when Biden made an appearance at the Shanksville Fire Station in Pennsylvania. The photo showing Biden wearing a "Trump 2024" hat, was shared by the Trump War Room X account with the caption, "Thanks for the support, Joe!"

Then-White House Senior Deputy Press Secretary and Deputy Assistant to the President Andrew Bates quickly addressed the incident on his X account shortly after the post went viral.

"At the Shanksville Fire Station, @POTUS spoke about the country's bipartisan unity after 9/11 and said we needed to get back to that. As a gesture, he gave a hat to a Trump supporter who then said that in the same spirit, POTUS should put on his Trump cap. He briefly wore it," Bates wrote.

Harris was reportedly shocked by the incident as she was less than a month from Election Day.

"What is he doing," she asked her team, according to the book via the Daily Mail. "This is completely unhelpful. And so unnecessary."

Harris' team decided to no longer campaign with Biden moving forward, attempting to emphasize her own leadership qualities while distancing herself from the then-president. Biden, however, told his supporters "we got to lock him up" while referring to Trump and called his supporters "garbage," which Trump then used to troll him with a photo-op of himself driving a garbage truck and comparing the campaign to his 2016 opponent Hillary Clinton.

"That’s terrible. Remember Hillary? She said 'deplorable.' And then she said 'irredeemable, right?…’ Trump said during a rally via the Daily Mail. 'That didn’t work out. ‘Garbage’ I think is worse, right?"

Harris was defeated by Trump who became the first Republican candidate since George W. Bush in 2004 to win the popular vote.

Armstrong & Getty