"When he said behind closed doors that 47 percent of the country considers themselves victims who refuse personal responsibility - think about who he was talking about: folks on Social Security who've worked all their lives, veterans who've sacrificed for this country" and "people who are working hard every day, paying payroll tax, gas taxes, but don't make enough income."
"I want to fight for them," he said.
Within the debate's first six minutes, Obama had twice accused Romney of saying things that were "not true."
Romney, who had dominated the first debate two weeks ago, often appeared to be on the defensive, accusing Obama of distorting his positions. He repeatedly made efforts to return the debate to his strongest point - Obama's economic record.
"The president has tried, but his policies haven't worked," he said, citing the nation's continued high unemployment rate. "That's what this election is about."
The sharply worded exchanges bore out predictions that Obama would more aggressively challenge Romney than he did in their first debate. After a distinctly lackluster performance in the first debate, which left Democrats demoralized, Obama was under great pressure to try to create a turnaround.
As the debate became increasingly heated, the moderator, CNN's Candy Crowley, several times had to admonish the candidates to stay on topic.
As they answered questions from audience members, each man aimed comments at voters that are key to their election efforts.
Obama offered a long list of policies that his administration has designed to help women in the workplace and said Romney's plans would deprive many working women of contraceptive coverage on their health plans, something Romney denied.
He also stressed his support for the bailout of the automobile industry in 2009 and Romney's opposition to it, which has been a key issue in Ohio and other Midwestern battleground states. Romney's stand would have cost "a million jobs," Obama said.
Romney stressed his desire to help small businesses and reduce their taxes and government regulations. "I know what it takes" to help the economy, he repeatedly declared.
Romney asked Obama why he hadn't delivered on his pledge to get comprehensive immigration reform through Congress - a grievance often voiced by Latino activists. "When the president ran for office, he said that he'd put in place, in his first year, a piece of legislation" to reform the immigration system. "He didn't do it."
Obama countered by reminding listeners of Romney's positions on a range of immigration issues unpopular with Latino voters, including opposition to the Dream Act, advocacy of "self-deportation" and support of Arizona's strict law against illegal immigrants, which allows law enforcement officers to ask people for identification documents if they stop them for other purposes and believe they may be in the country illegally.
"If my daughter or yours looks to somebody like they're not a citizen, I don't want - I don't want to empower somebody like that," Obama said.
The contentious debate covered virtually all the hot-button issues of the campaign, including the recent attack in Libya in which U.S. Ambassador J. Christopher Stevens and three other Americans were killed.
Obama said he had ordered the State Department to "beef up our security and procedures" in Libya and elsewhere, and had called for a full investigation of the nature of the attack and an all-out effort to hold those responsible accountable.
He also spoke in personal terms about the lives lost, before leaning on his broader foreign policy resume - mentioning the death of Osama bin Laden, the end of the war in Iraq and the wind-down of the U.S. role in Afghanistan as evidence that he means what he says.
"I am ultimately responsible for what's taking place there, because these are my folks. I'm the one who has to greet those coffins when they come home," he said.
He then sharply criticized Romney for his comments in the immediate aftermath of the attack. "You don't turn national security into a political issue. Certainly not when it's happening," he said.
Romney countered by pointing to mixed signals from the administration about whether the attack was a spontaneous demonstration in response to an anti-Islam video or a planned attack.
"It was a terrorist attack. And it took a long time for that to be told to the American people," he said.
But Romney's line of criticism ran into trouble as he insisted that Obama had not referred to the attack as an act of "terror" until two weeks after it took place.
Obama responded that he used the word "terror" to describe the attack the day after it occurred, in an address from the Rose Garden. When Romney attempted to dispute that, Crowley stepped in to say that Obama was correct.
In answer to a question about how he would separate his policies from those of George W. Bush, who remains unpopular with many voters, Romney said Bush had failed to balance the budget, but he would.
Obama responded that "George Bush didn't propose turning Medicare into a voucher. George Bush embraced comprehensive immigration reform. He didn't call for self-deportation." On some issues Romney has "gone to a more extreme place" than Bush, Obama said. "And I think that's a mistake."
Another exchange, over Romney's budget plan, illustrated how aggressively the president sought to drive home his campaign's criticisms of Romney.
Romney offered a broad outline of his proposals, saying the goal was to simplify the tax code and ensure that the middle class pay less.
"Under the last four years, they've been buried, and I want to help people in the middle class. And I will not - I will not under any circumstances - reduce the share that's being paid by the highest-income taxpayers," he said.
Obama responded with a multi-pronged attack, invoking his rival's 14 percent tax burden - "when a lot of you are paying much higher" - and recalling Romney's pledge to Republicans in the primary phase of the campaign that he would also seek to lower taxes on the top 1 percent. And, Obama said, Romney's budget math "doesn't add up."
"When he's asked, how are you going to do it, which deductions, which loopholes are you going to close, he can't tell you," Obama said. "We haven't heard from the governor any specifics, beyond Big Bird and eliminating funding for Planned Parenthood, in terms of how he pays for that.
"If somebody came to you, Governor, with a plan that said, here; I want to spend $7 or $8 trillion, and then we're going to pay for it, but we can't tell you until maybe after the election how we're going to do it, you wouldn't have taken such a sketchy deal. And neither should you, the American people," Obama said.
Romney also leaned on his resume, saying that throughout his career - as a businessman, running the Olympics, and as governor of Massachusetts, he always balanced the budget. Obama's record is one that "puts us on a road to Greece," he said.
"When we're talking about math that doesn't add up, how about $4 trillion of deficits over the last four years, 5 trillion," he said.
The race between the two men has been extremely close and static most of the time since late spring. Because of that general lack of movement, the two times the race has shifted significantly have taken on outsized importance.
The first shift took place during and just after the Democratic convention. With the help of former President Bill Clinton, Obama pulled wavering voters over to his side and took a small, but significant, lead nationally and in virtually all the battleground states. That lead grew a bit larger after Romney suffered from a series of mistakes that generated two weeks of bad news for him, most notably the public release of the "47 percent" videotape.
Then came the second shifting point, the initial debate. Voters who had previously been on the fence said they found Romney forceful and persuasive. Perhaps just as importantly, Romney's performance revived enthusiasm among Republican partisans, while Obama's lackluster demeanor and failure to respond forcefully to his challenger demoralized some Democrats.
The impact quickly showed up in polls as Romney picked up the support of Republican-leaning waverers. As before, the movement was not huge - an average of three to four points - but it erased most of the lead Obama had enjoyed, to return the race to the near-tie that had prevailed most of the summer and to send many of the president's supporters into a deep funk.
As the second debate approached, Romney appeared to have taken at least a small lead in at least one major battleground, Florida, and he was pouring resources into others, particularly Ohio and Virginia, in the hopes of establishing a clear path toward a majority in the Electoral College. Obama appeared to be holding on, at least for now, in several key states.