Their advisers have also represented widely differing positions. Randy Sheunemann, McCain's campaign foreign policy coordinator, seems to be mainly concerned with the threat from Iran and the security of Israel. In 2002, he was the founder of the Committee for the Liberation of Iraq, a pressure group which pressed for the overthrow of President Saddam Hussein.

McCain is surrounded by other prominent pro-Israeli neoconservatives such as William Kristol, editor of the Weekly Standard; Robert Kagan, a co-founder of the Project for the New American Century, which contributed greatly to the formulation of President George W. Bush's policies of unilateralism and military preemption; and James Woolsey, the former hardline CIA director.

Obama, in contrast, has been advised by men such as Zbigniew Brzezinski and Anthony Lake, both 'liberal' former national security advisers, who have warned of the dangers of attacking Iran and have been critical of some Israeli policies.

Brzezinski in particular has said that Obama would provide "a new definition of America's role in the world" – a diplomatic way of saying that Bush's policies have been profoundly misguided.

What seems clear is that the 'Bush Doctrine' – particularly the ambition to reshape the Middle East by force to make it safe for America and Israel – will be tossed into the waste bin of history.

But how valid in today's world are the differences between the two camps? America is going through a period of immense turmoil. Bush's two mandates have sapped America's strength – politically, militarily, morally and financially. The current firestorm engulfing America's financial institutions is only the latest sign heralding the end of America's international dominance.

Whoever wins next month's presidential election will inherit a much-diminished America. Its freedom of action will be greatly curtailed. The prime task of the next president, whether it is Obama or McCain, will be to repair the damage and rebuild America's strength.

In practice, therefore, it looks as if events will force the two candidates to move closer together. The ideological differences between Obama and McCain will be reduced. Already, they have both lent their support to the $700 billion bailout plan for the American banking system. On economic and social policy they cannot diverge too widely because the yawning budget deficit must inevitably greatly restrict their domestic spending plans.

On Iraq, too, their differences are more rhetorical than real. McCain has said that the surge has worked and that America should stay until "victory." But the truth is that everyone – including McCain – has come round to Obama's view that U.S. combat troops must be withdrawn sooner or later, and preferably sooner.

The only real policy difference between them is on the question of whether the United States should keep military bases in Iraq. Obama is against permanent bases, whereas McCain favors a long-term presence, on the model of the U.S. military presence in Germany and South Korea. This will, of course, depend on what the Iraqis will accept.

"Bomb, bomb, bomb, Iran!" McCain once sang frivolously at a public meeting, strongly suggesting that he would launch a military strike against Iran's nuclear facilities. But he has since had to temper his bellicosity. What with the unfinished business in Iraq and the worsening situation in Afghanistan, few American political or military leaders now believe that their bankrupt country – and their overstretched military – could seriously contemplate opening a third front against Iran.

Obama has advocated a dialogue with Tehran, something McCain previously dismissed. But McCain has now come round to Obama's position by saying that he would favor direct talks with Iran at secretary of state level.

On Afghanistan – now a key U.S. preoccupation – both Obama and McCain recommend sending in more troops. Obama has argued that the Iraq war was a mistake and that American effort should be redirected at rooting out al-Qaida from both Afghanistan and Pakistan. On this subject, both Obama and McCain are almost certainly mistaken. Afghanistan needs a political settlement not more war – even if it means political concessions to the Taliban.

Pakistan, in turn, needs to be saved from the profound destabilization being caused by American pressure for Pakistani action against the Taliban and by American military operations across the Afghan-Pakistan border.

Where the candidates diverge most profoundly is over the struggle against Islamic radicalism. McCain – echoing Bush – has said that radical Islamic extremism threatens American security and must be combated wherever it rears its head. Obama, in great contrast, is anxious to send the message that the United States is not at war with Islam.

McCain will want to pursue the 'Global War on Terror' by military means. Obama will want to defuse the terrorist threat by resolving political conflicts – primarily the long-running Arab-Israeli conflict.

This lies at the heart of their differences. McCain has declared himself to be a 'Zionist' and will – if his advisers are to be believed – support Israel whatever it does. In practice, this could mean no territorial concessions to the Palestinians, and no division of Jerusalem. Obama also supports Israel but, as he has explained, this does not mean embracing the views of the Likud.

Unlike McCain, Obama seems to understand the urgency for the United States to rebuild bridges with the Arab and Muslim world. That is why he has pledged to make the Arab-Israeli conflict one of his early priorities. But it remains to be seen whether these goals will survive the pressures and constraints of power.

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Patrick Seale is a leading British writer on the Middle East, and the author of "The Struggle for Syria"; also, "Asad of Syria: The Struggle for the Middle East"; and "Abu Nidal: A Gun for Hire." Copyright © 2008 Patrick Seale – distributed by Agence Global