U.S. President Barack Obama and Republican nominee Mitt Romney will clash in their final debate tonight over foreign policy, a topic that usually favours the incumbent but could pose some challenges to the president as controversy continues to swirl over the administration's handling of the deadly consular attacks in Libya.

"The incumbent always has a natural advantage in foreign policy because he's been dealing with it on an ongoing basis for more than three years and as a result is more deeply knowledgeable," Kathleen Hall Jamieson, a professor and director of the Annenberg Public Policy Center at the University of Pennsylvania, told CBC News. "It is easier as a result for the incumbent to catch the challenger making naive assumptions or misstating facts. It's also easier to box the challenger in any exchange. The advantage goes to the person with more context."

Although viewership is not expected to reach the same levels as the previous two debates, the debate at Lynn University in Boca Raton, Fla., will be the last opportunity for the two candidates to reach such a wide audience with the Nov. 6 election approaching. With polls still suggesting a tight race, Obama and Romney are seeking to capitalize on a successful debate performance and reach that contingent of undecided voters who would give their respective campaigns the edge needed for victory.

In the eyes of most observers, Obama redeemed his lacklustre first debate performance in his second matchup against Romney last week. Polls have suggested that following the first debate, Romney's support surged. Many pundits, including some conservative commentators, gave Obama the edge in the second debate, but it's unclear how much damage Obama was able to undo and whether it was enough to stop Romney's rise.

Both candidates head into the final debate with a number of Americans already having voted through early and absentee balloting that has been going on for weeks.

"So a last debate has less potential impact than a first debate," Jamieson said. "The impact is going to be less because there are fewer people available to make up their mind."

The topics of tonight's debate include:

• America's role in the world.
• Afghanistan and Pakistan.
• Israel and Iran, the changing face of Middle East and the new face of terrorism.

Many of those topics may play out as they did during the vice-presidential debate.

Romney will challenge Obama on setting a firm 2014 withdrawal date of troops from Afghanistan and criticize him for not being tough enough on Iran to stop its nuclear ambitions.

"Obama will cover a laundry list of achievements over four years in every region of the world, but naturally he will stress the ending of the war in Iraq, the winding down in Afghanistan, and – did I mention this? – the killing of Osama bin Laden," Larry Sabato, professor of politics and director of the University of Virginia Center for Politics, told CBC News via email.

But Romney is expected to try to undo some of the self-inflicted damage he suffered over his handling in the last debate of a question related to the Sept. 11 consular attacks in Benghazi, Libya, in which four Americans, including Ambassador Chris Stevens, were killed.

Source of controversy

It's an issue in which the president is considered vulnerable and that has become a source of controversy for the White House. Washington has faced criticism for ignoring requests to beef up security prior to the attacks and has also been accused of misleading the public about the cause of the attacks.

The administration at first suggested the attack was caused by an angry mob reacting to an anti-Prophet Muhammad YouTube video. They eventually claimed it was an al-Qaeda-inspired terrorist attack, but critics say the White House knew immediately the cause but did not want to acknowledge that al-Qaeda might be making a resurgence. (On Friday, the Associated Press reported that the CIA station chief in Libya reported to Washington within 24 hours of last month's deadly attack on the U.S. Consulate that there was evidence it was carried out by militants.)

However, in the first debate Romney focused on whether the president had called it a terrorist attack in a speech he gave the day after the Benghazi raid. Instead of using the issue to his political advantage, he got fact-checked on the spot by CNN moderator Candy Crowley, who backed up Obama who had told the town hall debate audience members that he had indeed given the attack the terrorism label.

Most agreed, including conservative commentators, that Romney had fumbled away an opportunity to strike a blow against Obama.

"Romney has make-up duty on Libya," Sabato said. "He did not capitalize on the Middle East conflict the way he should have."

But Jamieson said Romney's handling of the consular issue may mean he's hesitant to raise it again in this debate.

"When you make an assertion that someone didn't say something that they actually did say and you're called out on it by the moderator and, subsequent to the debate, by the fact checkers, you walk back into that terrain with some hesitance," Jamieson said.

"Obama's strongest statement in the debate was his statement about 'I'm the commander chief, I'm the president and I find that notion that we'd play politics offensive.' The president always has the high ground on those kind of issues if he knows how to claim it. And he claimed it."

(The issue of whether Obama did or didn't call it a terrorist act has become a point of contention. While he did refer in his speech to "acts of terror," critics say he didn't specify Benghazi and was instead referring to the 9/11 attacks.)

Romney's main task

However, Jamieson said Romney's most important task now is to try to win over swing voters, in particular moderate women — a demographic both he and Obama have been reaching out to in recent days.

"Moderate woman in general on security issues like presidents who are strong and decisive. Obama appeared that way in the last debate," Jamieson said.

"There's a threshold the challenger has to pass to have the country assume that he's an acceptable potential commander-in- chief. He needs to establish that he commands detail, he has a clear vision and that he is not vulnerable to the Democratic charge that he is going to be quick to take the country into war."

Democrats have repeated that charge about Romney, who has to be extremely careful not to reinforce that idea, Jamieson said.

"The dangerous allegation out there about Romney is that he seems too eager to keep us in wars we're already in and get into wars we're not in already," she said. "For practical purposes these two candidates don't differ that much on foreign policy. Romney just creates the illusion that he does."