Obama had a stern look. "No one is more concerned about their safety than I am,’’ he said.

Though Secretary of State Hillary Clinton this week said she took responsibility for the administration handling of the attack, Obama said that he was ultimately responsible. "I’m the president and I’m always responsible,” Obama said.

Romney blamed Obama for his administration’s constantly fluctuating explanation of the attack and said the president was too busy campaigning to fully deal with it. For days, senior officials had painted the attack as in tandem with mob violence that broke out in other parts of the Islamic world that day in response to an anti-Muslim video.

“The suggestion that anybody on my team would play politics or mislead when we’ve lost four of our own, governor, is offensive,” Obama said.

On domestic issues, Romney argued that the nation needs to be more energy independent, and he explained his support of more aggressive exploitation of domestic oil, gas, coal and other resources, as well as approval of the Keystone XL oil pipeline from Canada. "We’re going to bring that pipeline in from Canada," Romney said. "How in the world the president said ‘no’ to the pipeline I’ll never know."

Obama supports federal investments in clean energy research, but he backs only limited offshore drilling and has rejected the Keystone pipeline for now, though has said he will reconsider.

"We’re drilling more on public lands than the previous administration, and the previous president was an oil man," Obama said.

The candidates also sparred over taxes. Romney, who has proposed a 20 percent across-the-board income tax rate cut, vowed that higher-income people would pay no less than they do now, while middle-class consumers would get a break.

“And I will not – I will not under any circumstances, reduce the share that’s being paid by the highest income taxpayers. And I will not, under any circumstances, increase taxes on the middle class. The president’s spending; the president’s borrowing will cost this nation to have to raise taxes on the American people. Not just at the high end,” Romney insisted.

Obama scoffed. “Gov. Romney has a different philosophy,” he said.

The president explained his view, with a jab at Romney. “So what I’ve said is, your first $250,000 worth of income, no change. And that means 98 percent of American families, 97 percent of small businesses; they will not see a tax increase. I’m ready to sign that bill right now,” Obama said. “The only reason it’s not happening is because Gov. Romney’s allies in Congress have held the 98 percent hostage because they want tax breaks for the top 2 percent.”

Republicans have consistently blocked Democratic efforts to impose higher taxes on the wealthy.

Congress’ bipartisan Joint Committee on Taxation has found that even if most popular deductions were eliminated, such changes would only fund a 4 percent income tax cut.

The candidates battled over immigration. Romney in January urged illegal immigrants to “self-deport,” and he said in Iowa during his caucus campaign he would veto the DREAM Act, which would provide a path to citizenship for some young immigrants.