President Joe Biden briefly wore a red Trump campaign hat at an event commemorating the 23rd anniversary of the 9/11 attacks.

He was visiting firefighters in Pennsylvania near where one of the four hijacked planes crashed, and the White House said he donned the cap as a gesture of unity.

Video recording the incident shows the president having a friendly conversation with a Trump supporter before they swap headwear.

Donald Trump’s campaign was quick to see the funny side. “Thanks for the support, Joe!” one post read with a photo of Biden in the trademark Republican hat.

Another said “Kamala did so bad in last night’s debate, Joe Biden just put on a Trump hat”.

The event in Shanksville commemorated Flight 93, which was taken over by al-Qaeda hijackers in 2001. Passengers fought back and prevented the plane being crashed into a government building in Washington. All 40 aboard died.

Biden greeted firefighters and paid respects at a cross made from the broken fuselage.

Footage inside the fire station posted to social media shows Biden chatting with people and interacting with a man wearing a red hat with “Trump 2024” emblazoned on the front.

Biden offers the man a hat with the presidential seal on it, saying he would autograph it.

After some joking about his age, Biden hands the man the autographed cap and offers to trade it for the Trump hat.

Several of those gathered call on him to put it on, to which he shouts: “I ain’t going that far!” before putting it on, to loud cheers.

The White House later explained it was a way to underline the unity theme of the day.

“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,” deputy Press Secretary Andrew Bates wrote on X, formerly known as Twitter.

The daughter of the man who swapped hats with the president said Biden kept the Trump hat after the interaction.

“My pap don’t care who it is,” Kelsey Simmers told the BBC. “He likes to joke and have fun.”

She said that her father ended up getting another cap from the Trump campaign after they reached out.

The incident happened hours after Trump had joked on the debate stage about sending Vice-President Kamala Harris a Make America Great Again hat.