Biden added: “One of the dark days in history. The idea that wasn’t an insurrection — I don’t understand.”

June 6 commemorates the allied invasion of Normandy during the Second World War.

More than 2,500 American soldiers were killed during the invasion.

Biden was taking aim at Trump just moments before his latest gaffe.

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He said: “We’ll never forget him wanting tear gas to be sprayed on peaceful protesters outside the White House, and holding the Bible upside down. Now he’s writing his own Bible.”

The 46th President also slammed his predecessor over his handling of the coronavirus pandemic.

The fundraiser was held in the backyard of Douglas and Zeta-Jones’ riverfront home, where about 100 people sat under a tent to listen to both Douglas and Biden speak.

The latest blunder comes as 86 per cent of American voters believe Biden is too old to sit in the Oval Office amid concerns over his cognitive state.

Biden, who will turn 86 by the end of a potential second term, has made a series of errors but the White House has tried to play them down.

The 81-year-old was forced to step in after special counsel Robert Hur’s report that highlighted the president’s “significantly limited” memory over a probe into the 46th President’s classified documents.

“My memory is fine,” he insisted in a White House briefing. Biden gave an emotional response to a claim he could not recollect when his son died, saying: “How the hell dare he raise that?”

He added: “Frankly when I was asked the question, I thought to myself, was none of their damn business.

“I don’t need anyone to remind me when he [Beau Biden] passed away.”

The inquiry also concluded Biden “wilfully retained and disclosed” classified files but decided not to charge him.

“We have considered that, at trial, Mr Biden would likely present himself to a jury, as he did during our interview of him, as a sympathetic, well-meaning, elderly man with a poor memory,” the report added.