DES MOINES, Iowa — Republican presidential hopeful Mitt Romney, clearly irritated by a handful of hecklers amid supporters at the Iowa State Fair, insisted today that “corporations are people,” a comment Democrats gleefully predicted would be a defining moment of his campaign.

Hours before he was to face most of his primary opponents in an Iowa debate, the former Massachusetts governor was outlining options for reining in the federal deficit and overhauling entitlement programs. He acknowledged that raising taxes on individuals was an option, but he said he opposed it.

That’s when about a dozen hecklers started shouting at him.

“Corporations! Corporations!” they said, seemingly suggesting that corporations should take the brunt of new taxes.

“Corporations are people, my friend,” Romney said with uncharacteristic pique.

Several people in the front of the crowd — they identified themselves as linked to the liberal Iowa Center for Community Involvement — interrupted: “No, they’re not.”

Romney, who had already faced tough questions from the group’s members who arrived early and claimed the best seats, plowed right ahead.

“Of course they are. Everything corporations earn ultimately goes to people. Where do you think it goes?” said Romney, a wealthy businessman who has struggled with an aloof and elitist image as he tries for the GOP presidential nomination a second time.

His critics were not buying it and shouted back that the money goes in corporate pockets.

“Whose pockets? People’s pockets? Human beings, my friend,” Romney said.

Typically unflappable, Romney grew agitated as he kept calling on members of the group. After one question, he asked the questioner which group he was from and not where he lived, acknowledging the questions were planted to embarrass him.

Democrats didn’t hesitate to seize the moment.

“It is a shocking admission from a candidate — and a party — that shamelessly puts forward policies to help large corporations and the wealthiest Americans at the expense of the middle class, seniors and students,” said Democratic National Committee Chairman Debbie Wasserman Schultz, a Florida congresswoman.

Romney’s terse back and forth was just the latest example of why his advisers are limiting his public events. When he ran four years ago, he packed his days with events and weighed in on every topic. Now the front-runner, he instead prefers more controlled environments where he is less prone to gaffes or moments that could spin out his handlers’ control.

Romney faced other questions along the same lines during an appearance before the evening debate in Ames. One person asked about closing “corporate tax loopholes on big banks to raise revenue and balance the state budget.”

Romney’s reply: “There are a lot of people who use the word ‘loophole’ to say, ‘Let’s just raise taxes on people.’ And that I will not do. I will not raise taxes.”

His hecklers did not relent: “Pay your fair share.”

Romney’s comments hearken back to other candidates’ truthful — though politically embarrassing — moments on the campaign trail. During 2008, for instance, Sen. John McCain declared “the fundamentals of our economy are strong” as the economy melted down.

Earlier, Sen. Hillary Rodham Clinton drew fire during her Democratic primary for defending lobbyists.

“A lot of those lobbyists, whether you like it or not, represent real Americans,” she said. “They represent nurses, they represent social workers, yes, they represent corporations that employ a lot of people.”

It did little to endear her to her party’s base.

And in 2004, Sen. John Kerry told a crowd as he ran for president that he actually did vote for a bill paying for the wars in Iraq and Afghanistan “before I voted against it.” The line cemented the public’s view of the Massachusetts senator as a flip-flopper.

For Romney, his strident defense of corporations could leave the public little doubt about his loyalties.

“We don’t want to raise taxes on the American people. We don’t want to grow government, because government is too large already. We want to restrain the growth of government,” he said. “And when it comes to Social Security and Medicare and Medicaid, the truth is the promises we are making 20-year-olds, 30-year-olds and 40-year-olds have to be promises we can keep.”

His performance demonstrated a frustration with part of the crowd that would not relent.

Romney pointed angrily at the crowd and told them to give him a chance to answer. After a minutes-long exchange with Romney and the group shouting over each other, Romney said: “If you want to speak, you can. But it’s my turn.”

During another exchange, Romney dryly scolded his heckler: “The way this is going to work is that you get to ask your question, I get to give my answer. If you don’t like my answer, you can vote for someone else.”