The Man Who Would Be King: but the issue for Brown is when?

When the Rover car arrives at the school in south Wimbledon, Gordon Brown springs out before Tony Blair and dives into the crowd of carefully vetted Labour activists. "Thanks for all your help," he says warmly.

An hour and a half later, after a press conference, Mr Blair boards his battlebus after saying goodbye to the same group of party workers. But a beaming Mr Brown lingers a little longer, mobbed in the scrum. He shakes every hand as the joyous activists cling on to him for a few more precious seconds. Eventually, he joins Mr Blair on the bus for the next stop on their tour of marginal seats.

On the campaign trail, Mr Blair and Mr Brown have not only become inseparable. They have almost become indistinguishable. A visitor from abroad, who did not know which one was Prime Minister, could not have told from Monday's appearance at Merton Park Primary School. It could have been either man.

The image of a joint leadership has been projected across the country. Mr Brown often speaks first at rallies. In Shipley, West Yorkshire, he stood directly behind the Labour candidate Chris Leslie, who did the introductions, while Mr Blair seemed pushed to one side. When he speaks to the Labour faithful, Mr Brown repeatedly uses the phrase: "On behalf of Tony and myself."

As one Labour official who has travelled round with them put it: "You sense that a transfer of power is taking place before our very eyes. The centre of gravity is moving. Gordon is unstoppable. The succession will be a shoo-in. I don't think anyone in the Cabinet will even stand against him. The only question now is when it happens."

When well-wishers in the party ask the Chancellor's travelling aides how "the succession" is going, they get a pretty dusty answer: "We've got a general election to win first." An ally insisted there was "no deal" with the Prime Minister about a handover. Referring to Mr Brown's previous disappointments, he added: "We've had enough private deals. This time the deal is being played out in public."

On the road, Mr Brown often appears more relaxed than Mr Blair. He looks a more rounded politician than in the past, as he speaks about how becoming a father has confirmed his political instincts. "I think we recognise what I individually now know - that there are millions of parents really struggling quite hard to balance all their responsibilities - the help with their children and the demands of working life," he says.

The Chancellor's brow is certainly less furrowed than the Prime Minister's and he looks less tired. We have seen Smiling Gordon rather than Grumpy Gordon. There is a good reason. Mr Blair cannot be sure that he will win the election convincingly enough. Mr Brown knows he should emerge as a winner whatever the result. If Labour wins well, he will get the credit for acting as Mr Blair's human shield and ensuring that the party's campaign focused on the economy. It will only be a matter of time before the prime minister-in-waiting receives his rightful inheritance. If Labour's majority is cut sharply, it will be Mr Blair's fault. The Chancellor will stand to inherit the crown sooner rather than later, perhaps in a year's time.

Mr Brown has been the pivotal figure of the entire election. Not only has he engineered a "win-win" outcome for himself, he has seen off his enemies in Mr Blair's inner circle who wanted him to be dumped as Chancellor after the election. The idea seems laughable now, yet it was being urged on Mr Blair by some allies only a matter of weeks ago. "Gordon is a master tactician," said one close friend. "He has outmanoeuvred both the Tories and the Blairite ultras who wanted to sideline him."

It is significant that Alastair Campbell, who had a difficult relationship with Mr Brown while he worked in Downing Street, was the catalyst who enabled the two men to reforge their bond. Like Philip Gould, the Prime Minister's personal pollster, Mr Campbell realised that Mr Blair's relative weakness meant that he needed Mr Brown's strength.

After the worst year in their 22-year partnership since they first entered Parliament, their convincing public show of unity in the past four weeks has surprised even close allies. It may even have surprised them: Mr Blair had an emotional lump in his throat when Mr Brown gave him his strongest-yet backing over Iraq last week and said he would have done the same as Prime Minister.

For some Labour insiders, that was the moment when they knew the new deal between the two men was for real. When Mr Brown agreed, rather grudgingly, to play a frontline role in the campaign, some party strategists were sceptical. At first, the body language looked a little contrived. But then the joint leaders seemed to grow genuinely closer together. "It reminds me of a psychologist saying that if you force yourself to smile enough, you start to feel much better," said one Labour source.

By the final week of the campaign, the Blair-Brown body language seemed more natural. Mr Blair was just as likely to talk about the economy as Mr Brown, who in turn was just as likely to talk up Mr Blair's public service reforms. They were even finishing each other's sentences in joint interviews and press conferences. Yesterday they even sent out a last-minute "joint e-mail" to Labour waverers. In Gillingham, Mr Blair introduced Mr Brown as "my friend, our Chancellor, a fantastic asset for our country".

However convincing the double act, the question preying on the minds of cabinet ministers is: will it last beyond today? "We just don't know - ask me on Friday," said one. People in both camps believe there is no reason why what started as a marriage of convenience should not become a permanent union. The Brown camp wants the unity to continue into and beyond what it calls "the transition". But people on both sides admit there is no guarantee it will hold.

In return for taking a frontline role, Mr Brown ensured a share of campaign action for acolytes such as Ed Balls and Douglas Alexander. But he will not be banging on Mr Blair's door tomorrow to demand his followers get plum jobs in the post-election reshuffle. Again, the time for private deals has passed. An ally of Mr Brown explained: "There's only one thing that matters - when is Tony going to quit?"

Most Labour folk agree the answer depends on the size of the Labour majority. If he wins big, Mr Blair will have a chance of staying for something close to the three-and-a-half years that would be his dream scenario. If he wins a small majority, his tenancy in Downing Street could be much shorter. It could also cause a revival of tension between the two men. The public love-in of the past month could look like a very odd interregnum - indeed, a cynical act to fool the voters. If Mr Blair starts to lose the confidence of Labour MPs, it is even possible that a leadership election could be triggered - perhaps by a left-wing MP - and that Mr Brown would enter the contest against Mr Blair.

Despite all the warm words, there were some small differences between the Prime Minister and the Chancellor over tactics during the campaign. That they have not leaked out is a tribute to the discipline they both judged necessary.

The Brown camp had been warning about a Liberal Democrat advance before the campaign began. Mr Brown, a more tribal figure, has never had much love for the party he derisorily calls "the Liberals". But Mr Blair, whose mentor was Roy Jenkins, has always been reluctant to attack a party he regards as part of the social democratic family. As a result, the Labour attack was left very late, and may not stop a last-minute surge by Charles Kennedy's party.

Mr Blair wanted to take on the Tories on their strongest ground - immigration - early in the campaign. Mr Brown advised caution, saying Labour needed first to ram home its strongest message - on the economy - before invading enemy territory. Mr Blair took his advice.

Some Blairites thought it would have been better to stick to the "£35bn Tory cuts" charge rather than also accuse the Conservatives of having £15bn of uncosted spending commitments. But the Brown camp, anxious to pay back the Tories for the attacks they launched on Labour when it was in opposition, insisted that the two-pronged approach worked, showing a party whose figures did not add up.

Overall, these differences were not big and the new-found unity was maintained. "Despite all the problems of the past year, there was less tension during the campaign than in 1997 or 2001," said one insider.

There have been occasional reminders of the bad times which saw Mr Brown supplanted as the party's election supremo by Alan Milburn last September. On a train to Gillingham on Monday, an interviewer asked Mr Brown about his alleged insult to Mr Blair: "There's nothing you could ever say to me now that I could ever believe." Intriguingly, Mr Brown did not deny it.

"I have been asked that so many times in so many interviews. I also said I wasn't going to answer that question again because I have answered it so many times," he said.

Although Mr Blair acknowledged the political necessity of having Mr Brown alongside him, he has occasionally shown signs of irritation at his dependency on his Chancellor. He seemed taken aback on Sunday when Sir David Frost asked him at the start of a BBC interview: "You have come out solo?" As if he were surprised Mr Blair was allowed out without Mr Brown.

Mr Blair later reminded Sir David that there is "only one Prime Minister". The phrase is used by Blairites who acknowledge Mr Brown's contribution but insist their man can emerge from the election strong enough to secure his legacy before standing down in the autumn of 2008. They add that, whatever the result today, Mr Blair will retain the "power of patronage".

That may be true, but it may be a diminishing asset. By announcing last autumn that he would not fight a fourth general election as Labour leader, Mr Blair may have avoided difficult questions during this campaign about going "on and on". But one question will dominate British politics if he wins a third term today: how long is Mr Blair going to "go on" for? And one very important person will think he deserves an answer.

Has the election campaigning had an impact on voters' intentions?

NAGEELA YUSUF, 24

Student of Political Science from London

Voted Labour 2001

Will vote: Liberal Democrat

"I have been really detached from the campaigning and I made my decision by looking at the policies. Although I agree with most of Labour's policies, I don't like the leadership of Tony Blair so I will be voting Liberal Democrat. None of the three main parties has a comprehensive policy that would benefit someone like me."

BRENDA RIDEOUT, 77

Retired telephonist from Lancashire,

Voted Labour 2001

Will vote: Labour

"The campaign has not changed my mind. I have already voted Labour (by post) because it is the only party that does anything for working people. Tony Blair has held his own in this campaign. I think Michael Howard is smarmy and Charles Kennedy is a wimp. The war in Iraq has nothing to do with this election. It should be forgotten."

SOURMA ALAM, 24

A project co-ordinator from Bristol

Voted Labour 2001

Will vote: Labour

"I was undecided before the campaigning began but one of the reasons I am voting Labour is because of local issues. I work in the voluntary sector and I think Labour are good in Bristol. Nationally, despite everything about the war in Iraq, I think they deserve a second chance as the economy is in a good state."

AISHA GILL, 32

Lecturer in criminology at Roehampton University,

Voted Labour 2001

Will vote: Liberal Democrat

"The campaign has affected me. I have voted Liberal Democrat (by post). Iraq and the findings of the leaked Attorney General report was important. I could not bring myself to vote for a government that took this country to war on such minimal information. What else do we not know about other issues such as health and education?"

SHRUTI DUDHIA, 22

Sabbatical officer at Essex University

Voted Labour 2001

Will vote: Liberal Democrat

"I have come to this conclusion after going to debates with the local candidates. To get the student vote you can't rely on advertising, you have to go directly and engage with students. I will be voting for the Liberal Democrat candidate. He came across as a man you could trust and was very keen on funding for local colleges."

JAMES AUBREY, 21

Student in Cardiff

Too young to vote in 2001

Will vote: Probably Conservative

"I'm about 80 per cent sure that I'll vote Conservative. Michael Howard's difficulty is that he has a tendency to get personal; to call Blair a 'liar' seems irrelevant because all politicians lie! He is most effective when he focuses on policies. I wasn't impressed by Labour and there has been little campaigning locally."

IHTISHAM HIBATULLAH, 33

Independent media consultant from London

Voted Labour 2001

Undecided

"The issue of how Tony Blair and Michael Howard have not shown the slightest remorse over the war has made me angry. Both have shown they do not pay much attention to voters' interests, specifically the Muslim community.My own vote will be for a smaller party."

SHEILA GRIFFITHS, 88

Retired principal lecturer in human movement studies

Voted Conservative 2001

Will vote: Conservative

"The campaign has in no way changed my point of view but I wish it could have included an element of realism: the Liberal Democrats emphasised no tuition fees, knowing they couldn't achieve this if they ever got into government. The media should be helping people to uncover the truth."