Why are we asking this now?

The 59-year-old New York senator and former First Lady announced last week that she is joining the contest for the 2008 Democratic nomination. Her move is historic on two counts. Other women have tried - Pat Schroeder for the Democrat nomination in 1992 and Elizabeth Dole for the Republicans in 2000 - but none has had as good a chance of reaching the Oval Office - and never has the spouse of a former president attempted to win the top job in her own right. At this admittedly early stage, she is the clear front-runner in an already crowded Democratic field. A Washington Post/ABC News poll at the weekend gave her 41% against 17% for her closest rival, Illinois senator Barack Obama.

Why did she declare so early?

In this unpredictable contest, everything is happening earlier. Bill Clinton did not formally launch his victorious 1992 campaign until September 1991. Hillary is not the first, but the sixth Democrat (and fourth US senator) to declare. The first candidates' debate takes place in New Hampshire in April, nine months before the state holds its traditional first-in-thenation primary. Next year's primary calendar is more front-loaded than ever, so that the contest will probably be over by the beginning of March at the latest.

Candidates will thus need to have organisations and financing in place quickly. In 2008, it will be virtually impossible for a late entrant who suddenly acquires what George Bush senior famously called the "Big Mo", to mount a viable campaign.

Who are Hillary's biggest rivals?

Hillary's hand was partly forced by Obama's announcement earlier in the week that he was forming an exploratory committee, ahead of his official campaign launch on 10 February in Springfield, Illinois, at the home of Abraham Lincoln. Every sign is that the eloquent, charismatic Obama, 14 years her junior, will be a very strong candidate, but don't write off John Edwards, Democratic vice-Presidential candidate in 2004, who has been campaigning for the 2008 nomination ever since. Two heavyweight senators, Chris Dodd of Connecticut and Joe Biden of Delaware are also running, as is Bill Richardson, the former ambassador and current governor of New Mexico. John Kerry, the defeated 2004 nominee, may also enter the race. Hillary - more disciplined and better organised than her husband - is leaving nothing to chance.

What are Hillary's strengths?

Even Hillary's opponents, Democratic and Republicans alike, concede that she will be a formidable candidate. She has name recognition, vital at this early stage.

She has money in hand, a powerful fundraising machine at her disposal, topclass advisers (many from Bill's 1992 and 1996 campaigns), not to mention her landslide Senate wins in 2000 and 2006.

She has been a highly effective senator, and has tacked to the centre on many issues, notably national security, to shed her previous (unfair) image of an ultra-liberal "Madame Mao".

She has a supreme grasp of policy issues and a mind to match her husband's (she came top of her class at Yale Law School, where the couple met, while he managed "only" fifth).

Even her weaknesses may prove to be strengths. Hillary is forever associated with the collapse of her efforts in 1993 to devise a universal healthcare scheme.

Guess what? Just as she launches her campaign, universal healthcare is once more a hot issue in domestic politics.

What are her weaknesses?

In many ways, they mirror her strengths.

If anything, the fact that Hillary is a woman is probably a plus. But she can come across as too cerebral, lacking her husband's ability to connect emotionally with voters. She can also seem super-calculating, a politician who changes positions less out of conviction than opportunism. That factor could play into the hands of Obama, who will portray himself as a "new" politician, unsullied by the partisan political fray.

A further drawback is the Iraq war, which she supported in the crucial Congressional vote of October 2002. To the disappointment of liberals, she has thus far refused to disavow that vote.

Then there is Bill: spouse, informal adviser-in-chief, and the most gifted natural US politician of his era. None has a better sense of the electorate - but none is more scandal-prone. If he is caught playing the field again, not only would it be a huge distraction to Hillary's campaign, it would remind voters of the marital psychodrama in the Paula Jones/Monica Lewinsky sagas.

That is why even some Democrats fear she is too divisive to be elected.

So what role will Bill play this time around?

A tantalising question. As popular now as George W. Bush is unpopular, the 42nd President is a colossal asset (as long as he behaves himself). In 1992, candidate Bill Clinton told campaign rallies that if they put him into the White House, voters would get "two for the price of one".

Hillary is unlikely to make the point as explicitly now, simply because everyone knows it is true. Expect Bill to fundraise and campaign on her behalf - but probably not with her. On a speaker's rostrum, Bill overshadows everyone, including his wife.

He will play a key role behind the scenes, advising, encouraging and if necessary comforting. In public he will be making the case for Hillary as only he can.

What happens next?

A lot. Technically, Hillary has only set up an exploratory committee. This gives her a second bite at the publicity cherry when she officially launches her candidacy, doubtless with a rousing speech at a symbol-laden venue. In spring come the first debates, at which her main task will be to avoid gaffes .

Simultaneously, the vital "invisible primary" - the contest for funds and topclass organisers - will unfold. Summer and autumn will probably see more debates, and a host of straw polls ahead of the mid-January Iowa caucuses that kick off the primary season in earnest.

Six weeks later it will be all over - and Hillary will be either the first-ever woman to win a major party nomination, or just another failed candidate. But one thing is certain: she will never have a better chance than now.