IRVIN, Pa.--Mitt Romney, faced with mounting criticism from fellow Republicans over his refusal to release additional tax returns, sidestepped the issue on the stump Tuesday, seizing instead on what he sees as President Obama’s attack on free enterprise and success.

Speaking to a crowd of about 700 people at Horizontal Wireline Services, Romney said that Obama is “changing the nature of America,” and denigrating success and achievement.

“He said this: ‘If you’ve got a business, you didn’t build that. Somebody else made that happen,’” Romney said, quoting Obama from a campaign stop in Virginia last week. “That somebody else is government in his view.”

Republican presidential candidate and former Massachusetts governor Mitt Romney speaks at a campaign rally at Horizontal Wireline Services on Tuesday, in Irwin, Pennsylvania. Romney is campaigning Tuesday and Wednesday in the battleground states of Pennsylvania and Ohio as speculation on his vice presidential candidate continues to build. (Photo by Win McNamee/Getty Images) (Win McNamee/GETTY IMAGES)

“I find it extraordinary that a philosophy of that nature would be spoken by a President of the United States,” Romney said.

In a conference call Tuesday morning, former New Hampshire governor John Sununu (R), suggested that Obama didn’t understand how the American economy works.

“I wish this president would learn how to be an American,” Sununu said, offering a clarification later. “What I thought I said but what I didn’t say is the president has to learn the American formula for creating business.”

Romney and Sununu’s comments come as the Boston-based campaign has rolled out a new front, assailing Obama as a “crony capitalist” who hands out money to the businesses of campaign contributors.”

Democrats have insisted that the presumed Republican nominee is attempting to change the topic—Romney’s record at Bain as well as his tax returns have dominated the news cycle for weeks.

In suggesting that Obama has tried to “denigrate success and achievement,” Romney is trying to make the argument that he is being attacked because of his wealth, an argument that he has made before.

“President Obama attacks success and therefore under President Obama we have no success,” he said. “And I will change that.”

Romney made no direct mention of Bain Capital nor his tenure as governor of Massachusetts during his 20-minute speech.

His challenge is chipping away at Obama’s likability numbers—a June Pew poll gave Obama a 30-point edge over Romney when registered voters were asked who connects better with ordinary Americans.

At a fundraiser in Jackson, Miss. on Monday night, Romney worked to back against the suggestion that Republicans merely want to help the rich, an argument that Democrats are making as they push for ending Bush-era tax breaks for families making over $250,000.

“We’re accused, by the way - in our party -- of being the party of the rich. And it’s an awful moniker, because that’s just not true. We’re the party of people who want to get rich,” he said. “And we’re also the party of people who want to care to help people from getting poor. We want to help the poor.”

He also acknowledged a class divide, vividly on display at a high-dollar fundraiser where tickets prices topped out at $50,000.

“I know that people in this room are probably doing relatively well, relative to folks across this country. But not everyone in America is doing so well right now,” he said. “It’s tough being middle class in America right now. The waiters and waitresses that come in and out of this room and offer us refreshments they’re not having a good year.”

Romney has struggled to define himself as Democrats have poured millions into advertising in swing states like Pennsylvania where Obama leads 45 percent to 39 percent in the latest poll.