UPDATE 4-Obama, Romney Battle over Foreign Policy in Last Debate

(Updates with start of debate)

* Looking for an advantage with polls tied

* Can Obama fend off Romney challenge?

* Libya, Iran to figure prominently

By Steve Holland and Matt Spetalnick

BOCA RATON, Fla., Oct 22 (Reuters) - President Barack Obama and Republican challenger Mitt Romney battled over foreign policy on Monday in their third and final debate as they sought to break a deadlock in opinion polls heading into the final two weeks of campaigning.

The debate was the last major opportunity for either candidate to appeal directly to millions of voters - especially the roughly 20 percent who have yet to make up their minds or who could still switch their support at the Nov. 6 election.

World hot spots like Libya and Iran were likely to figure prominently, with Romney seeking to put pressure on Obama over what the Republican considers weak responses to the killing of the U.S. ambassador to Libya on Sept. 11 and Iran's nuclear ambitions.

The stakes are high in the 90-minute encounter at Boca Raton's Lynn University moderated by CBS News' Bob Schieffer.

The two candidates were tied at 46 percent each in the Reuters/Ipsos online daily tracking poll. Other surveys show a similar picture.

Obama came to Boca Raton with the advantage of having led U.S. national security and foreign affairs for the past 3 1/2 years. He gets credit for ending the Iraq war and the killing of al Qaeda leader Osama bin Laden in 2011.

But Romney will have many opportunities to steer the conversation back toward the weak U.S. economy, a topic on which voters see him as more credible. His goal was to appear as a credible alternative to Obama and avoid any gaffes that could deflate his recent surge.

Presidential debates have not always been consequential, but they have had an impact this year.

Romney's strong performance in the first debate in Denver on Oct. 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 Oct. 16, in what was deemed to be one of the most confrontational presidential debates ever, but that has not helped him regain the lead.

The viewership for the third debate could be lower than the others, since foreign affairs is not typically a priority for most voters and the two candidates were competing with a professional football game and a baseball playoff game on other channels.

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. A new Quinnipiac/CBS poll shows Obama leading by 5 percentage points in the Midwestern state, but another by Suffolk University shows the two candidates tied there. (Editing by Alistair Bell and Peter Cooney)