By Kevin Anderson BBC News Online, Washington

Three years after the attacks of 11 September 2001, and concerns about national security and terrorism top the list of voters' concerns in the United States.

Many voters see Bush as a better war leader

For weeks, the Bush and Kerry campaigns have traded attacks, trying to gain advantage in the opinion of voters on the issue of who will keep America safe.

Recalling Mr Bush's response to the 11 September attacks, the Republicans made national security central to their convention and to the president's re-election strategy.

John Kerry had hoped to blunt the Republicans' traditional advantage on national security by highlighting his military service in Vietnam.

But after attacks on his military record, Mr Kerry is changing strategy on the advice of former President Bill Clinton, trying to change focus to domestic issues like healthcare and jobs.

Bush attacks

President Bush and Vice President Dick Cheney have launched their most aggressive attacks against Mr Kerry's stand on national security in the wake of their party convention.

"When it comes to Iraq, my opponent has more different positions than all his colleagues in the Senate combined," Mr. Bush told a crowd in the battleground state of West Virginia.

Democrats will point out that John Kerry leads on Mr Bush when voters are asked about key domestic issues like health care and the economy.

But Mr Bush has enjoyed a consistently strong lead over Mr Kerry on the subject of the war on terror.

It is to the president's advantage to keep the focus on national security and to recall the period after 11 September when Americans rallied behind him.

Kerry reels

John Kerry has made his decorated military service in Vietnam the centrepiece of his campaign, hoping that would provide him a platform to criticise the president's conduct of the war in Iraq.

Kerry is having to change strategy less than two months form the polls

But attacks by a veterans' group on Mr Kerry's war record and his anti-war activities have eroded the advantage of his military service.

And Mr Kerry has had difficulty in criticising the president for going to war in Iraq when he voted for the resolution authorising force.

Also, voters are not making a distinction between the war on terror and the war in Iraq.

And polls show that, despite the 1000th US death in Iraq, people still feel that President Bush would handle the war there better than Mr Kerry.

Reeling in the polls from questions about his military service and Mr Bush's convention bounce, Mr Kerry has had to refocus his campaign, hiring members of Bill Clinton's political team.

And the former president himself encouraged Mr Kerry to reduce focus on his Vietnam service and shift the debate to domestic issues.

But this may prove difficult.

A recent poll by the Pew Center for the People and the Press found: "For the first time since the Vietnam era, foreign affairs and national security issues are looming larger than economic concerns in a presidential election."

The Pew poll conducted in August showed that 41% of respondents thought that war, terrorism and national security were the most important problems facing the United States.

However, the poll also found potential challenges for President Bush.