Nobody watching the climax of the US election campaign this weekend can be in any doubt that this is a dramatic and thrilling moment in American history. The prospect of electing the first black man to the presidency overshadows many of the dramas of the past, even the opportunity to elect a woman as president. If Barack Obama becomes the 44th president on Tuesday, it will symbolise a tectonic shift from a white Anglo-Saxon Protestant United States to a multi-ethnic America. Many would say “and about time”; the president should represent a racially mixed country whose population bears little resemblance to those who founded the nation in 1776.

However, before America and the world get carried away on a wave of euphoria, there are reasons why voters might pause. The first is that while opting for change, they are also choosing inexperience. Mr Obama would arguably be the most inexperienced occupant of the Oval Office in the postwar era. Only Jimmy Carter comes close and he had been a state governor.

That inexperience naturally extends to foreign policy. Mr Obama’s whistle-stop overseas tour in the summer took in Afghanistan, Iraq, Israel and parts of Europe. It also underlined how little the potential leader of the free world knows about it. Inevitably that will lead to misjudgments; he was prescient in warning of the risks of the Iraq war but wrong last year to oppose the successful troop surge.

Some of Mr Obama’s policy instincts are unpalatable, too. Like John Kerry four years ago he flirts with protectionism, promising to amend the North American Free Trade Agreement (Nafta) “so that it works for American workers”. A Democratic president allied to an overwhelmingly Democrat-controlled Congress could be a dangerous combination. There is much to be said for checks and balances. He favours redistribution through taxes, giving money to working families partly by expanding tax credits through a windfall tax on big oil companies and by tax hikes for the better off. If that sounds familiar, think Gordon Brown in 1997.

Nor should John McCain, his opponent, be written off so fast. A war hero, a maverick, a fighter, Mr McCain is a formidable politician combining amusing self-deprecation with hard-nosed common sense. Emotional and sometimes irascible, for the most part he has held his own in the televised debates against an opponent who is both preternaturally calm and eloquent.

Three things seem to be counting against McCain, although no election can be certain in advance. At 72 his age has been a disadvantage, making him look jaded against the youthful Mr Obama. That was compounded by his running mate. The choice of Sarah Palin, briefly seen as inspired if opportunistic, soon looked like an act of desperation. They hardly knew each other and were uncomfortable as a team. The fact that the vice-president is “a heartbeat away from the presidency” is even more significant because of Mr McCain’s age.

His biggest problem, however, has been fighting the election against the backdrop of George Bush’s presidency. While Mr McCain has sought to distance himself, it has been with only limited success and if anything he has been tugged to the right to secure heart-land Republican support. A presidency dominated for the most part by foreign policy disasters has ended in financial and economic disaster. Once the financial crisis went from Wall Street to Main Street - figures last week showed the sharpest quarterly drop in consumer spending for nearly three decades - a difficult task began to verge on the insurmountable. In contrast to Britain’s Labour government, America’s Republican administration has won few plaudits for its handling of the economic crisis. Some of that cynicism has rubbed off on Mr McCain, who has never looked comfortable dealing with the meltdown.

As it stands, it looks as if American voters will choose Mr Obama on Tuesday. In our view they will be making the right choice. There is a risk, of course. While we know a great deal about Mr Obama’s past, we know little of how he will act as president. And yet voters have to back their instinct and judge a candidate on his record and character.

In this gruelling campaign he has been determined, calm and measured - all important traits in a president. He has not made enemies needlessly and has learnt fast on the stump, bringing in a seasoned team of advisers. He has also headed a superb electoral campaign and, in the process, outwitted two of the most formidable electioneering forces in American politics - the Republicans and the Clintons. But perhaps most important of all, Mr Obama has that indefinable political magic - an ability to excite and inspire.

As he gets closer to the White House he has become more statesmanlike and measured, sounding less like a leftish protectionist and instinctive tax raiser and more like a potential president who knows he will have to restrain those pressures from within his party. Of course the new president will face enormous challenges. The scale of the banking crisis, and the measures taken in the dog days of the Bush presidency, will leave the nation’s finances hamstrung for years to come. Even if Mr Obama is a two-term president, he will still be dealing with the financial legacy of 2008 in 2016. For at least his first year in the White House he would be presiding over a painful recession.

He will also have to deal with a divided country, one that is ever more disillusioned with Washington and uncertain of how to face up to the challenges of the 21st century. At least he will provide a beacon for aspiring young African-Americans who are inclined to blame setbacks on racism. If Mr Obama wins, many will rightly think that anything is possible.

Perhaps surprisingly, being new on the international scene may help. He will encounter enormous goodwill, at least for a while. His prime aim will be to reinstate American leadership in ideas, to overcome the loss of moral leadership caused by the failure to think through the invasion of Iraq and the consequences of Guantanamo Bay. But many of the problems he will have to deal with remain daunting. How does he adapt a “war on terror” designed by Donald Rumsfeld and Dick Cheney without appearing to appease sworn enemies? How does he handle the morass of Afghanistan? The threat of a nuclear Iran? The new restlessness of Russia? The challenge of China and India to American power? The threat of climate change? These problems will test the finest American minds. One encouraging sign is that he seems likely to retain Robert Gates as defence secretary, an old hand and one of the few successes of the later stages of the Bush administration.

It is important, however, that if elected Mr Obama is not remembered just for being the first black president. That is hugely significant, of course, but it is not the reason for electing him. He promises a new start. If he fails to deliver, the sense of disillusion will be enormous. He deserves the chance and he has earned it. Mr Obama has the opportunity to bring change. Let us hope he can seize it.