If Barack Obama is elected president, sweeping into the White House with him will be two children, his daughters Malia, 10, and Natasha, 7. They will embark on an experience so extraordinary that it could throw a pall over the rest of their lives, says a man who knows what they will be facing.

Curtis Roosevelt is the grandson of President Franklin Delano Roosevelt, on whom Obama is now said to be modelling himself. At 78, he has written a poignant memoir of his childhood in the White House which describes how being “too close to the sun”, as he puts it, robbed him of self-confidence and ambition.

Born in 1929, the year of the Great Crash, Curtis paints a poignantly intimate picture of FDR and breaks Roosevelt family taboos by also focusing on the president’s disability (polio had left him without use of his legs) and beautiful mistress.

To his amazement, his grandfather’s name is now being conjured up almost every day as recession bites and commentators recall the visionary New Deal that FDR forged to combat the dire poverty of the 1930s.

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Nowadays, Curtis lives a world away from America’s election fever in Provence after a career at the United Nations (and three divorces). Does he have any advice for Obama’s children? “They’re a little older than my sister and I were when we moved into the White House, but they’re not too old to be affected by the same things that we suffered from. It’s not up to me to give advice, but I do think that Barack and Michelle are much more sensible parents than mine were.”

In the White House, Curtis was half of “a pint-sized double act” known as Sistie and Buzzie (his sister Eleanor, three years his senior, was responsible for nicknaming him Buzzie), which kept America entertained with photo sessions. “We had a marvellous life,” Curtis concedes, “but it was a life in which no one ever said that I had to do anything. This is devastating for a child: on the one hand, you live a life which is highly organised and highly controlled; and, on the other hand, you’ve got people who will run and pick something up if you drop it.”

The White House provided “a reality-rejecting sense of specialness”, he says. The whole Sistie and Buzzie act was “sick”, and constant public scrutiny left him highly self-conscious. “As a child, I ended up with an unrealistic view of what I was; I became addicted to a dream world, to a neurotic degree.”

He was also affected by “the expectation people had, based on the images of you they had seen in the press. You were constantly forced to adjust to those images, instead of developing your own self”. All the same, he remembers FDR as “wonderful, affectionate, full of fun”.

Aides would stand around FDR’s bed, laughing, as he had his breakfast every morning with Curtis and read the latest comic strips aloud. During outings, the president would order his motorcade to slow down so that he could point out ladies in funny hats to his grandson.

His favourite memory of FDR is “driving around with him in his special car at our estate, Hyde Park in the Hudson valley. He loved it so much, the mobility, because he was frustrated with his disability making him totally dependent on other people. When he got into that car, he wasn’t dependent on anyone because it was adapted for him. I’d watch in amazement as he accelerated and braked and shifted gears – all with his hands. He’d rush off and be so happy when he shook off his bodyguards”.

The president’s balance was so precarious that a strong gust of wind or an overenthusiastic handshake could topple him, but “I never saw FDR as physically weak, despite his disability. He was tremendously powerful in his arms and shoulders; he built his muscles up by doing exercises. He’d go down the driveway at our estate – it’s a good quarter of a mile long – on crutches. His legs were absolutely lifeless and he’d drag them along, throwing the crutches in front of him again and again”.

In contrast to the warm FDR, his wife Eleanor was reserved and sometimes abrupt. She would take Curtis to housing estates for the poor or welfare projects for destitute children. On a visit to London during the Blitz, she took him to see people sleeping in Tube stations.

Eleanor and FDR became surrogate parents to Curtis after their daughter, Anna, divorced his father. From then on, no photographs of his father were allowed in the White House. Even now, Curtis feels shame at the callousness with which, after a rare outing with his father, he would slip back into the presidential cocoon. Did he think he could be president one day?

“No, I realised I didn’t have what it took. My dream world was about being recognised as important; it started out as a child’s fantasy but then it shifted into a higher gear.”

Bright enough to get by without studying much, he failed to rise to any challenge in his studies or professional life. “I knew how to make an impression but I didn’t know how to follow it up with solid work. I went through a whole working career where I vacillated between lack of confidence and being pretentious.”

The rest of his family will almost certainly disapprove of his book. He says they consider intimate details concerning his grandfather’s disability and love affair to be as taboo now as they were during his lifetime.

After reading hundreds of family letters and documents, he believes that FDR’s long affair with the young and beautiful Lucy Mercer Rutherfurd may have started in 1915, two years after she was hired as Eleanor’s social secretary. In 1918, when FDR was being treated in hospital for pneumonia, Eleanor came across Lucy’s letters. Curtis says: “They clearly show FDR and Lucy were absolutely fascinated by each other, infatuated and in love. My grandfather would have been better advised to burn them.”

Eleanor descended into an anorexic depression and FDR promised her he would never see Lucy again. He not only failed to keep his word but two decades later asked his daughter Anna to organise his discreet meetings with Lucy at the White House. Anna agreed to do so behind her mother’s back.

Too Close to the Sun, by Curtis Roosevelt, is published by Public Affairs on Thursday at £17.99. Copies can be ordered for £16.19, including postage, from The Sunday Times BooksFirst on 0870 165 8585

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