U.S. President Donald Trump in the Oval Office in on Jan. 14.Evelyn Hockstein/Reuters

The cognitive decline of former U.S. president Joe Biden was undeniable, minutes into his 2024 debate with Donald Trump. He looked confused, bewildered, and couldn’t complete a thought. The people around Mr. Biden had clearly been aware that he was losing his faculties, which is why, for months prior, he had largely been hidden from public view. But that decision made the reveal especially striking: Instead of the public gradually acclimating to the President’s changing demeanour, it was presented with a President who was suddenly incapable of answering a simple question.

Mr. Trump has been a volatile, meandering, eccentric personality his entire life. In a 1991 book entitled Trumped! The Inside Story of the Real Donald Trump, former Trump employee John R. O’Donnell wrote that, as a business mogul, Mr. Trump was “a terrible communicator and didn’t know how to sort out his thoughts on a daily basis, let alone provide long‑term corporate direction.”

For decades, Mr. Trump has often gone off on tangents. During a purportedly motivational speech he gave at a business expo in Colorado in 2005, he railed against his ex-wives and former business associates, and told attendees to “be........

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