Leading article: Amid the tumult, it's the fight for the centre that will count

In this extraordinary US election year, two extraordinary political journeys are now one step from completion. Last week, Barack Obama became the first African-American to be anointed candidate of a major party. This week, the Republicans for their part embraced a man who made his career by fighting his party establishment. John McCain would be the oldest man ever to enter the White House should he triumph on 4 November. Above all, however, his nomination is proof that, if the Republicans are to win, they must tear up the rule book and start again.

These have been a riotous couple of weeks, from the latest instalment of the Clinton melodrama at the Democrats' convention in Denver, to the sensational debut on the national political stage in St Paul, Minnesota, of Sarah Palin, governor of Alaska only since 2006 and Mr McCain's surprise choice to be his vice-presidential running mate. But nothing has changed the underlying dynamic as this marathon campaign enters the final two-month straight up to election day.

Four years ago, George W Bush narrowly prevailed over John Kerry by using a "50 per cent plus one" strategy of getting out the vote of the Republican faithful, in the belief that core Republican support would exceed that of the Democrats. The approach worked then, but it will not work now. In 2004, the country was almost equally split between the two parties, but in 2008 Democrats lead in the so-called "generic" vote by 10 per cent or more. A shrunken Republican base is therefore no longer enough. This election will be decided where elections usually are, in the centre. To win, Mr McCain must persuade independents and some renegade Democrats that he is a better bet than Mr Obama.

His acceptance speech in St Paul was a first major step in that direction. True, much of it sounded like the standard Republican boilerplate of lower taxes, muscular foreign policy and countless allusions to faith, family and country. Mr McCain was also notably short on policy specifics. Unmistakably, however, he was aiming at undecided voters in the middle.

The night before Ms Palin, the self-proclaimed "pitbull in lipstick", did what she was supposed to do, by laying into Mr Obama and electrifying conservatives who have never much trusted Mr McCain. His right flank thus covered, the candidate himself was free to reach out to the centre.

First came the required mea culpa, with the acknowledgement that Republicans, so long in the driving seat in Washington, had "lost their sense of purpose". Next, Mr McCain cited specific cases of ordinary Americans struggling in these tough times. This Republican president would be different, he was saying; he would care about little people and understand their problems. Washington too would be a different, less bitter place in the brave new McCain/Palin era. He vowed to end partisan rancour by working across traditional party lines, and to bring independents and Democrats into his administration.

McCain's pitch

John McCain, as courageous as a politician as when he was held captive and tortured by the North Vietnamese, has a truly inspiring life story. But even in the theatrical, totally choreographed setting of a convention, it was hard on Thursday evening to suspend the disbelief that a man of 72 – who has been in Washington for a quarter of a century – could now be the change agent America so desperately needs. Mr McCain promises a new beginning, but to be brutally honest, he was showing every one of his years as he spoke. The figure he brought to mind was Bob Dole, another elderly war hero and Washington veteran who carried the Republican standard into the 1996 election. Alas, Mr Dole could never shake off the musty odour of the past, and was duly trounced by Bill Clinton.

A dozen years on, Mr McCain faces an even more difficult challenge. It is his own party that has held the White House for the past eight years, when the country is the gloomiest it has been in decades about its prospects. That the candidate mentioned Mr Bush only once in his speech, and even then not by name, was proof of how great an albatross the desperately unpopular President is to his own party. Tellingly, too, Mr McCain barely mentioned Iraq in his speech, despite the perceived success of the troop "surge" which he championed. As he well knows, the war is another guaranteed vote-loser for Republicans this autumn.

And then there is the economy, sliding ever deeper into trouble if yesterday's grim US unemployment figures are any guide. Barring a major foreign crisis, this will be a an election dominated by public discontent on the bread-and-butter issues of jobs, wages, mortgages and healthcare coverage. In normal circumstances, a Democratic victory would be all but guaranteed.

But it is not – and the reason is Barack Obama. His party may be far ahead, but polls suggest he and Mr McCain are tied in the presidential race. This election will not turn on the Republican's heroic war record, or on the fireworks of Ms Palin, or on the ability of Joe Biden, her Democratic opposite number, to connect with working-class voters in swing states such as Pennsylvania and Ohio.

In presidential elections, voters are choosing a president, the man who will lead their country, the man who will intrude on the TV screen in their living rooms in moments of national crisis. A failed convention may hasten defeat, but a successful one does not guarantee victory.

The make-or-break moments of this campaign will come in the candidates' debates. If the young Democrat can hold his own with the tested Republican on great matters of war, peace and economic struggle, he can lay to rest charges that he lacks the spine and the experience to sit in the Oval Office. If he does, Mr Obama will win, perhaps by a wide margin. If not then it is quite possible, even in the best year for Democrats in a generation, that he will lose.