(AP) - President Barack Obama sharply challenged Mitt Romney on foreign policy in their final campaign debate Monday night, saying, "Every time you've offered an opinion you've been wrong." The Republican coolly responded, "Attacking me is not an agenda" for dealing with a dangerous world.

Romney took the offensive, too. When Obama said the U.S. and its allies have imposed crippling sanctions on Iran to halt nuclear weapons development, the Republican challenger responded that the U.S. should have done more. He declared repeatedly, "We're four years closer to a nuclear Iran."

Despite the debate's stated focus on foreign affairs, time after time the rivals turned the discussion back to the slowly recovering U.S. economy, which polls show is the No. 1 issue for most voters.

They found little agreement on that, but the president and his rival found accord on at least one international topic with domestic political overtones - Israel's security - as they sat at close quarters 15 days before the end of an impossibly close election campaign. Each stressed unequivocal support for Israel when asked how he would respond if the Jewish state were attacked by Iran.

"If Israel is attacked, we have their back," said Romney - moments after Obama vowed, "I will stand with Israel if Israel is attacked."

Both also said they oppose direct U.S. military involvement in the efforts to topple Syrian President Bashir Assad.

The debate produced none of the finger-pointing and little of the interrupting that marked the presidential rivals' debate last week, when Obama needed a comeback after a listless performance in their first meeting on Oct. 3.

But there was no mistaking the urgency. The two men frequently sniped at one another even on issues where they agree, and reprised their campaign-long disagreements over the economy, energy, education and other domestic issues despite ground rules that stipulated the debate cover international affairs.

Obama and Romney are locked in a close race in national opinion polls. The final debate behind them, both men intend to embark on a final two-week whirlwind of campaigning. The president is slated to speak in six states during a two-day trip that begins Wednesday and includes a night aboard Air force One as it flies from Las Vegas to Tampa. Romney intends to visit two or three states a day.

Already four million ballots have been cast in early voting in more than two dozen states.