The first thing to know is that politics in New York defies definition. It certainly defies prediction. Woe be to the lambs who venture to the land of the oracle. Several months ago saw a headline in a British newspaper declaring "Why Hillary Will Not Win". The lure of the bold prognostication is understandable, but very perilous.

No one, for example, could have predicted that the race for the New York Senate seat would turn into the fiasco that it has in the last two weeks, nor would anyone have bet money on what happened in the last 24 hours.

When Senator Daniel Patrick Moynihan announced his planned retirement last year, the pundits responded with confidence that First Lady Hillary Rodham Clinton would not run for the seat, despite rumours that she was interested.

There was no way that Mrs Clinton, after years of sparring with the comparatively genteel Washington press corps, would thrust herself into the vortex of the piranha-like New York media, no way that she would step down from her dignified pedestal in the White House to wrestle in the mud of New York politics, they said.

Then Mrs Clinton announced she was a candidate. She was running, and she was running to win, the first First Lady to run for elective office. She began touring the state, bought a house in Westchester County, traded barbs with the press, and spent the last six months meeting voters in every God-forsaken corner of the state.

She kissed babies, ate breakfast at back road truck-stops, did not shy away from spirited discussions. Accompanying journalists who had expected a light schedule were taken aback.

One day, standing near Niagara Falls, a major and somewhat corny tourist attraction near Buffalo in upstate New York, Mrs Clinton turned toward the grizzled press corps that had criticised her often for being a stranger to the state, and asked how many of them had actually been to Niagara Falls before. Um . . . none of them.

"See New York. Travel with our campaign," she chortled.

As Mrs Clinton has been getting on and off buses, and has achieved even now her goal of visiting every one of New York State's 62 counties, her presumed opponent was running a very different campaign.

New York City Mayor Rudy Giuliani was always thought to be a formidable opponent to Mrs Clinton. He had, in American parlance, all the right stuff, especially in New York. A tough former federal prosecutor, Mr Giuliani was credited with cleaning up New York, cutting crime, standing up to unions, straddling the line between conservative and liberal positions, somehow getting the place to work.

He has done so at a price. Relations between the city's black community and the police have deteriorated, and a series of police shootings of unarmed suspects has been greeted by the mayor with a less than compassionate attitude.

But love him or hate him, that has been part of Mr Giuliani's appeal. He has always been anti-Clinton (as in Bill, the President), with no pandering to what he thinks voters want to hear, no governing by way of public opinion polls. Mr Giuliani has always been his own man, and that has been his strength with voters.

And then this.

The unravelling of Rudolph Giuliani began about two weeks ago. In a press conference unusual for its decorum, Mr Giuliani announced that he had been diagnosed with prostate cancer. He would weigh his treatment options in the coming week, and come to a decision about whether to run for the Senate sometime before the Republican party's nominating convention on May 30th.

The announcement took New York by storm, and eyes turned towards the personal side of Mr Giuliani, an area that has been discreetly off limits for years.

He and his wife, actress and sometimes television presenter Donna Hanover, had reportedly been estranged for the past three years.

The couple live at Grace Mansion, the mayor's official residence on New York's East side, with their two children. But there have been rumours that things were not well. The two met on a blind date in 1983 and were often an affectionate couple, especially during the early years at Grace mansion.

But unpublished rumours surfaced several years ago that Mr Giuliani was having an affair with an aide. The rumour reached print in Vanity Fair magazine in 1995, but both Mr Giuliani and the aide, Ms Cristyne Lategano, denied it. End of topic.

None the less, Mr Giuliani and his wife were no longer seen together in public. Mr Giuliani stopped wearing his wedding ring, though Ms Hanover continued to wear hers.

Shortly after the announcement of his cancer diagnosis, the New York Post published a photograph of Mr Giuliani and a companion named Ms Judith Nathan dining at a well-known Upper East Side restaurant. The next day, Mr Giuliani said Ms Nathan was "a good friend, a very good friend", and did not deny what many in the press had known for at least a year - that Ms Nathan was his girlfriend.

Things went downhill from there. At yet another press conference, Mr Giuliani announced that he and his wife were separating after 16 years of marriage.

Moreover, of Ms Nathan he added, "I'm going to need her more now maybe than I did before."

Trouble was, Mr Giuliani had failed to relay this to his wife before the news conference. Ms Donna Hanover had sent her children off to school, exercised, and was having lunch when the phone rang with friends asking if she had seen her husband on television announcing their separation. She had not.

It is difficult to imagine a more tawdry situation. A tearful Ms Hanover then called her own press conference and contended that the marriage had been in dire straits ever since Mr Giuliani's affair with his aide years earlier, the one he had denied.

But things were about to take another turn. On Thursday night, Mayor Giuliani sat for a televised town hall forum, answering questions about anything and everything. He said he had not yet made a decision about which prostate cancer treatment to undergo, nor had he made a decision about whether to run.

By Friday morning everything changed. Confounding the pundits again - many had said Mr Giuliani would never quit the race, even if he had to run while undergoing cancer treatment - the mayor announced he would not run. The decision was variously described as stunning and shocking, coming at the 11th hour, and throwing the Republican Party into chaos.

The likelihood is that the Republicans will nominate Congressman Rick Lazio on May 30th to run against Mrs Clinton. (Congressman Peter King is also interested in running, but concedes that he faces an uphill battle to wrest the nomination from Mr Lazio.)

And what a different race it will be. Mr Lazio, a former lawyer elected to Congress in 1992, has none of the star power that Mr Giuliani had. He has little of the wit or toughness. He has a record of conservative voting in Congress over which the Clinton campaign is salivating.

Still, Mr Lazio - or Santa Claus if he were to run against Mrs Clinton - will be a magnet for right-wing conservative support and money that is determined to keep Mrs Clinton out of the Senate. He is attractive and bright.

For her part, the First Lady is likely to do what she has done from the beginning of the campaign: stick to the issues, meet the voters, ignore the pundits, and understand that nothing is predictable in politics.