Barack Obama and John McCain will call a temporary truce to their increasingly bare-knuckled presidential battle today when they appear in New York to commemorate the seventh anniversary of the September 11 terror attacks.

For all the outward civility the issues of terrorism and the war in Iraq - central in the minds of voters - will be swirling beneath the surface 54 days before the general election.

The Democratic and Republican candidates will pay their respects at Ground Zero, the site of the World Trade Centre, and listen to the names of its 2,751 victims. They will then speak separately at a public service forum. Both campaigns pledged to suspend negative advertising for a day.

Although the faltering US economy is the dominant issue in this election the September 11 attacks still stand as the seminal event of the past eight years, the trigger of the wars in Iraq and Afghanistan, and the root of questions about what kind of judgment Mr McCain or Mr Obama would display.

“I believe that foreign policy may still be the top issue in the race - not because it is automatically the most important, given the state of the economy - but because national security is an area where voters realise the centrality of the role played by the president. It is also the area where McCain and Obama have many of their sharpest differences of opinion,” Michael O'Hanlon, a foreign policy analyst at the Brookings Institution in Washington, said.

The campaigns are aware that another terror attack before the election, or an event that brought the issue to the forefront, could alter the race significantly. Some analysts believe that such a moment would favour Mr McCain because of his foreign policy credentials and experience as a navy pilot and prisoner of war in Vietnam. In polls he has a healthy advantage over Mr Obama.

Many Democrats believe that the video appearance of Osama bin Laden four days before the 2004 election - in which the al-Qaeda leader threatened fresh attacks on the US - helped President Bush to win.

In the event of another attack, or the reappearance of bin Laden, Mr Obama will argue, as he done for months, that the Iraq war, which Mr McCain supported, distracted America from its true mission after September 11, 2001: dismantling al-Qaeda, finding bin Laden, and tackling the causes of Islamic radicalism.

The other unforeseen consequence of the Iraq war was the success of the surge of 30,000 additional US troops into the country. Eighteen months ago Mr Obama - and most Democrats - believed that the war would be a burning issue.

Not only has it faded as a dominant issue but Mr McCain is also trumpeting his support for the “surge” last year and Mr Obama's staunch opposition to it.

Referring to his support of the surge, Mr McCain said recently: “Senator Obama and I also faced a decision, which amounted to a real-time test for a future commander-in-chief. America passed that test. I believe my judgment passed that test. And I believe Senator Obama's failed.”

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