Donald Trump has challenged President Joe Biden to take a cognitive test – only to confuse the name of his own doctor.

The Republican presidential nominee has regularly lambasted his rival over his various gaffes, despite his own tendency to ramble or get facts wrong.

Speaking in Detroit, Mr Trump questioned the president’s mental acuity. He said: “He doesn’t even know what the word ‘inflation’ means. I think he should take a cognitive test like I did.”

Seconds later, he continued: “Doc Ronny Johnson. Does everyone know Ronny Johnson, congressman from Texas? He was the White House doctor, and he said I was the healthiest president, he feels, in history, so I liked him very much indeed immediately.”

However he was clearly referring to Ronny Jackson, who was the White House physician for part of his presidency.

He was elected to Congress in 2021 and is one of Mr Trump’s most vociferous defenders on Capitol Hill.

Mr Trump took the cognitive test in 2018 at his own request, Dr Jackson told reporters at the time. The exam is designed to detect early signs of memory loss and other mild cognitive impairment.

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The former president, who turned 78 on Friday, has made questioning whether the 81-year-old President Biden is up for a second term a centrepiece of his campaign.

But online critics quickly seized on his Saturday night gaffe, with the Biden campaign posting a clip of the moment minutes later.

President Biden poked fun at Mr Trump’s age too, wishing him a happy 78th birthday “from one old guy to another” on X.

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Joe Biden and Volodymyr Zelenskyy shake hands after the signing of a new security agreement this week. Pic: Reuters

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It comes after a week in which both rivals were criticised for awkward moments in public.

President Biden appeared to freeze for several seconds before briefly stumbling on his words at an event marking the anniversary of Juneteenth on Monday.

Meanwhile, a teleprompter malfunction in Las Vegas saw Mr Trump go off on a bizarre tangent about his dislike of sharks.