| BOCA RATON, Florida
BOCA RATON, Florida President Barack Obama and Republican challenger Mitt Romney face off in front of the cameras for a final time on Monday as opinion polls show their battle for the White House has tightened to a dead heat.
With 15 days to go until the November 6 election, the two candidates turn to foreign policy for their third and last debate, which starts at 9 p.m. (0100 GMT on Tuesday).
The stakes are high. The two candidates are tied at 46 percent each in the Reuters/Ipsos online daily tracking poll, and the debate will likely be the last time either candidate will be able to directly appeal to millions of voters.
Though few voters cite the war in Afghanistan or other national-security topics as a top concern, Obama can point to a number of successes on his watch, from the end of the Iraq war to the killing of Osama bin Laden.
Romney will use worries about the prospect of a nuclear Iran and turmoil in Libya to try to amplify concerns about Obama's leadership at home and abroad.
"Many voters are ready to fire Obama if they see Romney as an acceptable alternative," said David Yepsen, director of the Paul Simon Public Policy Center at Southern Illinois University. "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.'"
Presidential debates have not always been consequential, but this year they have had an impact.
Romney's strong performance in the first debate in Denver on October 3 helped him recover from a series of stumbles and wiped out Obama's advantage in opinion polls.
Obama fared better in their second encounter on October 16, but that has not helped him regain the lead.
The Obama campaign is now playing defense as it tries to limit Romney's gains in several of the battleground states that will decide the election.
Romney could have a hard time winning the White House if he does not carry Ohio, and a new Quinnipiac/CBS poll shows Obama leading by 5 percentage points in the Midwestern state.
More than 60 million viewers watched each of their previous two debates, but the television audience this time could be smaller as it will air at the same time as high-profile baseball and football games.
Much of the exchange, which takes place at Lynn University in Boca Raton, Florida, will likely focus on the Middle East. Other topics such as trade with China and the debt crisis in Europe could allow the candidates to circle back to the economic concerns that are topmost on voters' minds.
Campaigning in Canton, Ohio, Vice President Joe Biden on Monday reminded voters of Obama's pledge to pull troops out of Afghanistan in the next two years and pointed out that Romney and his running mate Paul Ryan have made no such guarantees.
"They said, quote, it depends. Ladies and gentlemen, like everything with them, it depends," Biden said. "It depends on what day you find these guys."
Romney accuses Obama of presiding over a weakening in U.S. influence abroad, but he has to assure voters he is a credible alternative to the president on the world stage. The former Massachusetts governor's July trip to London, Jerusalem and Poland was marked by missteps.
The two men at their second debate last week clashed bitterly over Libya, a preview of what is to come on Monday evening. They argued over Obama's handling of the attack last month on the U.S. consulate in Benghazi, Libya, in which Ambassador Chris Stevens and three other Americans were killed.
The Obama administration first labeled 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 campaign trip the day after the attack, has given Romney ammunition to use at Monday's debate.
"The statements were either misleading by intention or they were misleading by accident. Either way, though, he's got to get to the bottom of this," Romney adviser Dan Senor said on NBC's "Today" show.
Obama and his allies charge that Romney exploited the Benghazi attack for political points while officials were still accounting for the wellbeing of U.S. diplomats.
Regarding foreign policy overall, Obama's allies accuse Romney of relying on generalities and platitudes.
"It is astonishing that Romney has run for president for six years and never once bothered to put forward a plan to end the war in Afghanistan, for example, or to formulate a policy to go after al Qaeda," U.S. Senator John Kerry, the Democrats' 2004 presidential nominee, wrote in a memo released by the Obama campaign on Monday.
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. He also has faulted Obama for setting up a politically timed exit from the unpopular Afghanistan war, and accused him of failing to support Israel, an important ally in the Middle East.
The Republican challenger is likely to bring up a New York Times report from Saturday that said the United States and Iran had agreed in principle to hold bilateral negotiations to halt what Washington and its allies say is a plan by Tehran to develop nuclear weapons.
The 90-minute debate, moderated by Bob Schieffer of CBS, 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.
(Additional reporting by Matt Spetalnick, Mark Felsenthal and Susan Heavey; Writing by Andy Sullivan; Editing by Karey Wutkowski and Paul Simao)