There is only one question for the voters: Exactly who is Gordon?

When Tony Blair goes to Buckingham Palace, he will name the date of a constitutional innovation. For the first time, a party leader will go to the country saying that, if elected, he will give way to someone else before the end of the new parliament. The more the Flight-less Tories seem unlikely to win, the more the dominant question of the election campaign will be: "Who is Blair's Labour successor?"

When Tony Blair goes to Buckingham Palace, he will name the date of a constitutional innovation. For the first time, a party leader will go to the country saying that, if elected, he will give way to someone else before the end of the new parliament. The more the Flight-less Tories seem unlikely to win, the more the dominant question of the election campaign will be: "Who is Blair's Labour successor?"

Certainties have been settling over the political landscape in recent weeks like pollen. One certainty is that the Tories will not win; another is that Gordon Brown will be the next prime minister. After Michael Howard's misjudgement in deselecting Howard Flight as a Tory candidate, the first layer of certainty has grown. The second has followed, after a run of opinion polls suggesting that the Chancellor is the most popular politician in Britain.

Indeed, it is just possible that Brown could be prime minister in five weeks' time - if Labour's majority vanishes. The opinion polls, including our latest one today, still point to a majority for Blair of more than 100. But those who are prepared to put money on it in the spread-betting market predict a majority of just over 50. And Labour is vulnerable if it loses votes to the Liberal Democrats, as it usually does during election campaigns. No doubt Brown would prefer not to become prime minister in those circumstances - heading a minority government in a hung parliament, or with a tiny majority of, say, fewer than 20 seats. But at least his succession would be quick and uncontested.

The more likely scenario is that he takes over in two or three years' time, either when a crisis brings Blair down or when the orderly hand-over, cursorily sketched by the Prime Minister last year in his constitutionally innovative statement of intentions, comes about.

That is not totally certain, either. Blair may move Brown to the Foreign Office after the election, although he would still be heir-apparent. But the passage of time does strange things to people. And there is a breeze now blowing through Westminster gossip that is disturbing the dust. In recent weeks, the name of Alan Johnson has been mentioned several times as the one who might frustrate Brown's ambition at the last. He is the latest to fill the one position in recent British politics that has been even more brutally unkind to its holders than being leader of the Tory party - that of the "Stop Gordon" candidate. Johnson, a good minister who performs well in the Commons and comes across effectively on television, has two advantages over previous incumbents. One is that hardly anybody has heard of him. The other is that he has a sense of humour. The Secretary of State for Work and Pensions tells a joke against himself about a recent photocall with a group of happy pensioners. "Who is Johnson?" one asked.

However, the next prime minister is still most likely to be Brown, who was notably cheerful on Thursday at the final cabinet meeting before the election campaign starts. Fighting an election on Labour's economic record is, in the phrase that even the Prime Minister has taken to using, a no-brainer. But if Brown is the most popular politician in Britain, this is partly because he is a Rorschach ink blot in which people see what they want to see. The most important thing they do see is that he is not Tony Blair. He is the "None of the Above" candidate: the anti-politics candidate. We know that he is a politician really, but he is a useful proxy for that nebulous and powerful idea that things could be different. Thus he supported the invasion of Iraq, but it wasn't his idea. He managed to avoid being abused for the appointment of Paul Wolfowitz to the World Bank, despite being an alternate governor of its ruling body.

But if we are voting Blair and getting Brown, admittedly after some delay, are we, the electorate, not entitled to ask, like Johnson's confused pensioner: "Who is Brown?"

That was the question posed by Irwin Stelzer last Tuesday. It was an important question, not least because of the person who was asking it. Stelzer is director of economic policy studies at the Hudson Institute and a columnist for Times Newspapers. More significantly, he is a friend of Rupert Murdoch.

Stelzer and the British Chancellor have been courteous to each other, although in his columns Stelzer has often chivvied Brown to follow through in his professed admiration for the US economy. Last Tuesday, however, he was more direct. He may not have written The Times's headline "Why Brown is wrong for No 10", but his message was clear: that Brown would not make a good prime minister if "he continues to seek the future in his made-in-Scotland rear-view mirror". And he is a statist meddler who "does not yet understand how to equip his nation to play in the unforgiving game of international competition".

Two questions follow. One is whether Brown really would turn the clock back on the market-based reforms of public services that he has always given to understand that he opposed. I fear that he might, but you can never be sure what someone will be like in the top job until they get there. Who can guess what Brown, in the same circumstances, might have done about Iraq? If Brown does take over from Blair, will he not seek to reassure the economically liberal part of the New Labour coalition?

This leads on to the second question raised by Stelzer's article: to what extent would Brown try to rebuild Labour's relations with the press? Wooing the Murdoch papers and the Daily Mail has long been central to Blair's strategy. Murdoch still carries the torch for Blair, while the Daily Mail has gone sour. But the Mail remains curiously two-faced about Brown, who has good personal relations with Paul Dacre, the paper's editor. Dacre attended the funeral of Brown's daughter, Jennifer, three years ago. This produces some strange journalism. On Friday, a leading article in the Daily Mail, headed "Taxed to the hilt", included this sentence: "Gordon Brown is the one outstanding exception in an administration that is otherwise seen as incompetent and untrustworthy." I like that "seen as". By whom? Why? Not because of a sustained campaign of distortion and vilification by newspapers including the Daily Mail, surely?

If Brown loses The Sun, many Labour Party members and supporters will feel a sense of relief. But Brown is too canny a politician to turn his back on such an important part of the New Labour coalition. The Chancellor faces a tricky path over the next two years. He needs to try to keep The Sun and the Daily Mail on-side while maintaining his position as the heir- presumptive, which means retaining the support of Labour's electoral college, to whom The Sun and the Daily Mail are the enemy. That will test him. If he succeeds, he will deserve to become prime minister.