'He wouldn't have just faded away, but he certainly wouldn't have been the mega star he is today,' said Fred Siegel, a senior fellow at the Progressive Policy Institute and a one-time adviser to Mr Giuliani. 'The way he handled 9/11 was his defining episode.'

Republican polls place Mr Giuliani, 62, ahead of, or at least neck and neck with, the party's other prospective White House candidates.

And in a match-up against Democratic frontrunner Hillary Clinton for the 2008 presidency, Mr Giuliani would be the voters' choice, a survey for Fox News revealed last month, though he may also consider running as vice-president on a ticket with Arizona Senator John McCain.

Analysts say his popularity with voters has more to do with the way he led his city through its bloodiest crisis, inspiring the nation with his iron leadership and spirited eloquence, than with his position on political and social issues.

He is pro-choice, pro-gun control, and pro-gay rights. His first wife was his second cousin and he cheated on his second wife with the woman who would become his third, triggering a messy soap opera-style divorce. All qualities that would ordinarily be an ill fit with voters on the powerful religious right.

'But after [President George W.] Bush, competence is going to be at a premium - that's what people are going to be interested in more than everything else,' said Professor Siegel, author of The Prince of the City: Giuliani, New York and the Genius of American Life. 'And competence is Mr Giuliani's forte.'

Rudolph William Louis Giuliani was born on May 28, 1944, in Brooklyn, New York. His grandparents were Italian immigrants, four of his uncles were police officers and a fifth was a captain in New York's fire department. His mother, Helen, worked as a bookkeeper, and his father, Harold, was a plumber and bartender who had served 18 months in jail for robbing a milkman at gunpoint in 1934 - something Mr Giuliani claimed he never knew until it was reported by a newspaper six years ago.

Instead, he credits his father as a strong moral influence whose advice served him well in later life.

'My father used to say to me, 'Whenever you get into a jam, whenever you get into a crisis or an emergency and everybody around you is getting very excited, you become the calmest person in the room and you'll be able to figure your way out of it,' he revealed in a 2003 interview.

At school he was often more interested in baseball than books, yet ultimately chose to go to law school. He became a judge's clerk, then a prosecutor and, in 1981, associate attorney-general. But the post of New York mayor beckoned.

'I had prosecuted organised crime, white-collar crime, drug dealing, municipal corruption, other forms of government corruption. I got to see all the bad parts of the city and I knew all the good parts just from my life as a New Yorker. So I felt, well, this is an ideal situation for me. The city needs a reform mayor ... so maybe I could get myself elected.'

Which he did, serving two consecutive terms from 1994 until December 31, 2001.

He revived rundown neighbourhoods, got 700,000 people off the welfare rolls, and adopted a zero tolerance policy against crime, even taking on mafia operations including the notorious Gambino family.

Critics say that the get-tough policy resulted in an overzealous police department - there were jokes that New Yorkers were more afraid of the NYPD than they were of the criminals - and argued that the drastic drop in crime on Mr Giuliani's watch simply followed a national trend steered by other social factors.

His biggest battle was with prostate cancer, but there were also fights with the media and his own staff, a short temper, a scandalous private life, and public exasperation with his big ego and self-absorption.

By September 10, 2001, with the end of his second term just weeks away, Mr Giuliani was ready to slip into New York history.

But the following day, the nation saw a new side to the father-of-three as he became America's consoler-in-chief and held his city together, leading emergency workers and public officials through the ash and rubble of the World Trade Centre to set up a makeshift command centre and take charge.

In the early hours of the following morning, with images of the terror he had witnessed playing over and over in his head, he pulled out a biography of Winston Churchill and turned to the wartime prime minister's words: 'I have nothing to offer but blood, toil, tears and sweat.'

His leadership won him an honorary knighthood from Britain's Queen Elizabeth, the prestigious Reagan Medal of Freedom, and saw him named Time magazine's Man of the Year 2001. When his term as mayor was up at the end of that year, he was known as one of the country's most respected voices on counterterrorism and emergency management.