John Rentoul: Portents of mortality that drive on an ambitious man

It will affect him but not perhaps in ways that people expect. If anything, it may increase his sense of impatience, the feeling that he has a lot to do and not much time. For whatever the doctors say about how the heart condition is not life-threatening and how it may not even be related to stress, it is bound to act as a reminder that he is, after all, mortal.

Tony Blair's political career was born of portents of mortality. It was his father's stroke at the age of 40, when Tony was 11, that helped to instil the drive to succeed. "After his illness, my father transferred his ambition on to his kids. It imposed a certain discipline. I felt I couldn't let him down."

His mother's death, from throat cancer, at the age of 52, just as Tony left Oxford University at the age of 22, was another jolt. "You suddenly realise - which often you don't as a young person - that life is finite, so if you want to get things done you had better get a move on."

He pursued political advancement with an outward smoothness that belied the urgency of ruthless ambition. As with Margaret Thatcher, the idea of taking things easy does not register, even if he is more grounded in the diversions of a young family.

He is confident, proud even, of his physical strength. An irregular heartbeat may be a memento mori, but he is constitutionally unsuited to regarding it as an injunction to slow down. Although he comes across on television as a rather slight figure, he is 6ft 2in, with broad shoulders, muscular arms and about the right weight, just under 13 stone, for peak fitness.

He stopped smoking on the day of his wedding when he was 26, and has never suffered anything more serious than a cold.

He has always blandly ignored, in public, the suggestion that his father's medical history might have hereditary implications. Unlike his mother's cancer, many of the risk factors for strokes can be inherited. And Blair also accepts that his father's stroke was brought on by overwork. Leo Blair senior was not only a law lecturer at Durham University, but a barrister and an active local politician, looking for a winnable Conservative seat at the time. Only once, in an interview to mark his 50th birthday earlier this year, did Tony Blair accept that his father's stroke might mean he ought to be careful: "Yes, I suppose so."

His family history is otherwise good. His natural grandparents on his father's side lived into their 80s; his mother's mother lived well into her 70s, when she developed Alzheimer's, not usually hereditary; while his maternal grandfather died of appendicitis in 1924. Nor is he likely to be under any pressure to ease off or stand down from Cherie.

This week's flutter may have more effect on how people see him, adding another layer of ageing to former newness of Labour. A heart scare, especially if it does not recur, is not as visible as his receding and greying hair, or his glasses. But it is always there in the folk memory.

Many have forgotten that John Major, at 47, was the youngest prime minister for nearly 100 years when he came into office. But Blair was different because he made generational change a theme of his message of national renewal. His young family was on display in a way that Major's teenagers were not.

In 1997, Blair embodied the lifestyle to which so many aspired: smart, successful wife; Ford Galaxy full of attractive children; a shirtsleeved, meritocratic attitude to getting the job done. Blair is still in tune with the national mood and, with a three-year-old son, in touch with the preoccupations of people half his age.

John Rentoul is author of Tony Blair: Prime Minister, published by Time Warner Books.