Ronald Reagan, whose alleged lack of mental agility was the basis for scores of jokes, especially outside the US, was suffering from the early effects of Alzheimer's in his first term as president, according to his son.
In a new book, My Father at 100: A Memoir, Ron Reagan cites two examples. He recalls watching his father, then 73, debate 1984 Democratic presidential nominee Walter Mondale. "I began to experience the nausea of a bad dream coming true," he writes. "Some voters were beginning to imagine grandpa – who can never find his reading glasses – in charge of a bristling nuclear arsenal, and it was making them nervous. Worse, my father now seemed to be giving them legitimate reason for concern. My heart sank as he floundered his way through his responses, fumbling with his notes, uncharacteristically lost for words. He looked tired and bewildered."
Ron Reagan, the youngest of the former president's four children, says his father may have suspected the onset of Alzheimer's in 1986 when he was flying over familiar canyons north of Los Angeles and became alarmed that he could no longer remember their names. Reagan was diagnosed with the disease in 1994, five years after leaving office, at which point the jokes about his mental powers suddenly ceased to be funny, even to his detractors. The popular Republican president died in 2004 at 93 from complications of the disease.
Mr Reagan believes his father would have left office before his second term ended in 1989 had the disease been diagnosed then. "I've seen no evidence that my father (or anyone else) was aware of his medical condition while he was in office," he writes. "Had the diagnosis been made in, say, 1987, would he have stepped down? I believe he would have."
But Mr Reagan says that the issue of his father's health should not tarnish his legacy as the 40th president of the United States. He writes: "Does this delegitimise his presidency? Only to the extent that President Kennedy's Addison's disease or Lincoln's clinical depression undermine theirs."
He adds: "Better, it seems to me, to judge our presidents by what they actually accomplish than what hidden factors may be weighing on them. That likely condition, though, serves as a reminder that when we elect presidents, we elect human beings with all their foibles and weaknesses, psychological and physiological."
The Ronald Reagan Presidential Foundation denied that Reagan showed signs of the illness while he was in office. A statement from the foundation says: "This subject has been well documented over the years by both President Reagan's personal physicians, physicians who treated him after the diagnosis, as well as those who worked closely with him daily. All are consistent in their view that signs of Alzheimer's did not appear until well after Reagan left the White House."
Mr Reagan's book will be published on Tuesday, in time for the 100th anniversary of his father's birth on 6 February.