United States President Barack Obama and Republican challenger Mitt Romney face off in front of the cameras for a third and final time later today near the end of a campaign season marked by a high number of memorable debates.

With 15 days to go until Americans vote on November 6, the two candidates turn to foreign policy for their last encounter at Lynn University in Boca Raton, Florida. The 90-minute event starts at 2pm (NZT) and will be moderated by Bob Schieffer of television network CBS.

The stakes are high as the pair run neck and neck in the polls. Presidential debates have not always been consequential, but this year they have had an impact.

Romney was reeling from a series of stumbles when he entered the first debate in Denver on October 3, and his strong performance changed the course of the race, vaulting him back into an even position in the polls with Obama.

Democrats fretted openly about their candidate's timidity at the podium.

Then, Obama was ruled the narrow winner of the second encounter on October 16 when he got the better of Romney in a testy exchange over Libya. His campaign halted the slide but it was not enough to edge ahead in the polls. An NBC News/Wall Street Journal survey on Sunday put the two tied at 47 per cent among likely voters.

Several times, debates marked turning points in the Republican primary race. Texas Governor Rick Perry's White House run effectively ended when he failed to remember one of three federal agencies he would scrap, in his infamous "Oops" moment in a debate.

Newt Gingrich presented Romney a serious challenge at points during the primaries, partly on the back of strong debate performances.

The former House of Representatives speaker won fans among Republicans by attacking moderator John King of CNN for asking him about an old extramarital affair at the start of a debate in January in South Carolina. Gingrich won the state's primary days later.

LAST-CHANCE DEBATE

Monday's debate is the last major chance for Romney and Obama to be seen by millions of voters before Election Day. More than 60 million viewers watched each of their previous two encounters.

If recent history is any guide, it is anybody's guess as to how the third face-to-face session will play out.

Despite a reputation for being wooden, Romney has shown an ability to rise to the occasion and perform well on stage.

The incumbent Democrat seems to have the upper hand on foreign policy since he has been in charge of US national security for nearly four years. He gets credit for the mission that led to the killing of Osama bin Laden and for pulling troops from Iraq.

David Yepsen, director of the Paul Simon Public Policy Center at Southern Illinois University, said Obama has two goals: show that "he's got nothing to apologise for in the way he has conducted foreign policy" and question the inexperienced Romney on foreign affairs.

China could be an issue for Obama to raise, said Yepsen, as Romney has vowed to crack down on Chinese trade policies if elected.

"He's got to really pin Romney down on what he means by some of the things he's saying. 'What do you mean when you say you are going to get tough on China? How do you go about doing that?'" he said.

A former governor of Massachusetts whose trip abroad in July to London, Jerusalem and Poland was marked by missteps, Romney has to assure voters he is a credible alternative to the president on the world stage.

Romney accuses Obama of presiding over a weakening in US influence abroad.

"Many voters are ready to fire Obama if they see Romney as an acceptable alternative," said Yepsen. "Foreign policy has not been a big driver of this campaign but I think Romney could add some icing to his cake if people say, 'Hey, this guy is on top of world affairs.'"

The two men at their second debate last week clashed bitterly over Libya, a preview of what is to come today. They argued over Obama's handling of an attack on the US Consulate in Benghazi, Libya, in which Ambassador Chris Stevens and three other Americans were killed.

The Obama administration first labelled the incident a spontaneous reaction to a video made in the United States that lampooned the Prophet Mohammad. Later it said it was a terrorist assault on the 11th anniversary of the September 11, 2001, attacks.

This shifting account, and the fact that Obama went on a political fundraising trip the day after the attack, has given Romney ammunition to use at today's debate.

Critics have accused Romney of relying on generalities and platitudes - he has harkened back to Ronald Reagan's "peace through strength" doctrine - and he could be put on the spot if he resists providing specifics.

Romney has promised to tighten the screws over Iran's nuclear program and accused Obama of "leading from behind" as Syria's civil war expands and setting up a politically timed exit from the unpopular Afghanistan war.

The Republican is likely to bring up a report that the United States and Iran agreed in principle to hold bilateral negotiations to halt what Washington and its allies say is a plan by the Islamic Republic to develop nuclear weapons.

The debate will be divided into six segments: America's role in the world; the war in Afghanistan; Israel and Iran; the changing Middle East; terrorism; and China's rise.

- Reuters