WASHINGTON (AP):

With one debate left, United States (US) President Barack Obama and his challenger Mitt Romney are retreating from the campaign trail to bone up on foreign policy, leaving the work of courting voters to their running mates.

Tomorrow's debate in Boca Raton, Florida, with its focus on international affairs, is the third and final between the two rivals and comes just 15 days before the election.

Obama left last Friday for Camp David, the presidential hideaway in Maryland's Catoctin Mountains.

He was to remain there with advisers until tomorrow morning. Romney was to spend the weekend in Florida with aides preparing for the debate.

Tomorrow's 90-minute debate will be moderated by Bob Schieffer of CBS News.

It will be similar to the first debate, with both men standing at lecterns on a stage. Schieffer has listed five subject areas, with more time devoted to the Middle East and terrorism than any other topic.

While the economy has been the dominant theme of the election, foreign policy has attracted renewed media attention in the aftermath of the attack on the US Consulate in Benghazi, Libya, that killed four Americans, including US Ambassador Chris Stevens.

Obama had ranked well with the public on his handling of international issues and in fighting terrorism, especially following the death of Osama bin Laden.

Ad wars intensified

But the administration's response to the Libya attack and questions over levels of security at the consulate have given Romney and his Republican allies an issue with which to raise doubts about Obama's foreign-policy leadership.

Romney has spent large amounts of time off the campaign trail to prepare for the upcoming foreign policy debate.

Aides say the additional time preparing is well-spent even if it comes at the expense of public events.

Meanwhile, the ad wars intensified even more with the release of new TV spots for both sides.

Romney's latest ad criticises the president's policies on debt, health care, taxes, energy and Medicare. It echoes the argument Romney has made in the campaign's final month: The country cannot afford four more years of Obama in a number of areas, not just the economy. The campaign did not say where the spot would air.

And an independent group supporting Obama said it would begin airing ads that draw renewed attention to Romney's tenure at the helm of the private equity firm Bain Capital.