WASHINGTON — During a private reception with wealthy donors this year, Mitt Romney described almost half of Americans as “people who pay no income tax ” and are “dependent upon government.” Those voters, he said, would probably support President Obama because they believe they are “victims” who are “entitled to health care, to food, to housing, to you name it.”

In a brief and hastily called news conference Monday just after 10 p.m., Mr. Romney acknowledged having made the blunt political and cultural assessment, saying it was “not elegantly stated,” but he stood by the substance of the remarks, insisting that he had made similar observations in public without generating controversy.

The video of Mr. Romney, the Republican presidential candidate, was made in May, offering a rare glimpse of his personal views. Mr. Romney told reporters that he had been “speaking off the cuff in response to a question” at the fund-raiser, and said he wanted “to help all Americans — all Americans — have a bright, prosperous future.”

Democrats quickly condemned the remarks as insensitive, and Mr. Obama’s campaign accused Mr. Romney of having “disdainfully written off half the nation.”

The video surfaced as the campaign enters its final 50 days and as Mr. Romney sought to restart his campaign with new ads and new messaging, in response to calls in his campaign and from outside for him to be more specific about how his policies would fix the nation’s economy and help the middle class.

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Now, the video has raised the possibility that Mr. Romney’s campaign will be sidetracked, with attention focused again on his proposed tax cuts for the wealthy, the release of his personal tax returns and his ability to connect with middle-class voters. With its unvarnished language, the video seems to undermine what aides have argued is an enduring attribute that would appeal to independent voters: a sense that Mr. Romney is, at base, an empathetic and caring man.

Snippets of the video of Mr. Romney were posted online Monday afternoon by Mother Jones, a liberal magazine, which said it had obtained the recording and had confirmed its authenticity. The magazine said it was concealing the identity of the person who had recorded the video and the location and time of the recording.

The author of the Mother Jones article, David Corn, said on MSNBC that the video was shot on May 17 at the Boca Raton , Fla., home of Marc Leder, a financier, who held a $50,000-a-person fund-raiser for Mr. Romney that night.

In one clip, Mr. Romney describes how his campaign would not try to appeal to “47 percent of the people” who will vote for Mr. Obama “no matter what.” They are, he says, “dependent upon government, who believe that they are victims, who believe the government has a responsibility to care for them.”

He says those people “pay no income tax,” and “so our message of low taxes doesn’t connect.” Mr. Romney adds: “My job is not to worry about those people. I’ll never convince them they should take personal responsibility and care for their lives.”

The comments were much more stark than Mr. Romney’s usual remarks, though he typically talks in public about supporters of Mr. Obama’s wanting big government to take care of their problems. He often accuses Mr. Obama and his supporters of wanting to bring a European-style socialism to the United States . In the video clips, Mr. Romney says his campaign is concentrating on the “5 to 10 percent in the center” whom he described as “thoughtful” voters.

Mr. Romney addressed the video, somewhat awkwardly, at a fund-raiser Monday night in Costa Mesa, Calif., summoning reporters with a few moments’ notice to walk through the Segerstrom Center for the Arts, which was filled with guests sipping drinks at tables elegantly draped in blue cloths.

Mr. Romney said his comments addressed “a question about direction for the country: Do you believe in a government-centered society that provides more and more benefits? Or do you believe instead in a free-enterprise society where people are able to pursue their dreams?”

Asked whether he delivers different, starker messages to wealthy donors than he does to ordinary voters at campaign rallies, Mr. Romney said he was offering the same message, though he has never used the language in the video at a public event.

But Mr. Romney acknowledged that he wanted to offer donors a candid sense of his strategy, given the role they play in his campaign. “That’s something which fund-raising people who are parting with their monies are very interested in — knowing can you win or not and that’s what this was addressing,” he said.

Mr. Romney, who has been under fire for releasing only two years of his tax returns, was quickly attacked by the Obama campaign. Jim Messina, Mr. Obama’s campaign manager, said in a statement Monday evening that it was “shocking” that Mr. Romney would “go behind closed doors” to describe nearly half of the country in such terms.

Late Monday night, Mr. Messina sent out a fund-raising appeal to Mr. Obama’s supporters, saying that someone “who demonstrates such disgust and disdain for half of our fellow Americans” does not deserve to be president.

Mr. Romney is not the first presidential candidate to be caught speaking candidly at a fund-raiser. Four years ago during the Democratic primary campaign, The Huffington Post published Mr. Obama’s remarks at a San Francisco fund-raiser, saying small-town Pennsylvania voters, bitter over their economic circumstances, “cling to guns or religion or antipathy to people who aren’t like them” as a way to explain their frustrations.

The Romney video was unearthed apparently with help from James Carter, a grandson of former President Jimmy Carter . Mr. Carter, who lists “oppo researcher” on his Twitter bio, told New York Magazine that he had helped find the videos and get them to Mr. Corn. He is credited with “research assistance” on the Mother Jones Web site. Mr. Romney has repeatedly compared Mr. Obama to President Carter, suggesting both were failures.

In an audio clip posted online from the same fund-raiser, apparently by the person who gave the videos to Mother Jones, Mr. Romney is heard joking that he would have an easier time winning the election if his father had been born to Mexican parents.

“My dad, as you probably know, was the governor of Michigan and was the head of a car company,” Mr. Romney says in the audio clip. “But he was born in Mexico , and, uh, had he been born of, uh, Mexican parents, I’d have a better shot at winning this.”

But the most striking part of the video is Mr. Romney’s characterization of nearly half of the country. His assessment of the “47 percent” echoes a line of conservative thinking that is championed by his running mate, Representative Paul D. Ryan of Wisconsin . Mr. Ryan has long argued that nearly half of the people in America are either “dependent” or “reliant” on the federal government.

Mr. Romney’s figure of 47 percent comes from the Tax Policy Center, which found that 46.4 percent of households paid no federal income tax in 2011.

But most households did pay payroll taxes. Of the 18.1 percent of households that paid neither income taxes nor payroll taxes, the center found that more than half were elderly and more than a third were not elderly but had income under $20,000. Roberton Williams, a senior fellow at the center, wrote in a blog post last summer that about half of those were off the rolls because they had low incomes.

Michael D. Shear reported from Washington and Michael Barbaro from Costa Mesa, Calif. Michael Cooper contributed reporting from New York.

