IRWIN: Republican Mitt Romney shrugged off growing pressure on Tuesday to release more of his tax returns, and his campaign lashed out at President Barack Obama in an effort to turn the campaign debate away from Romney's business and financial record.

Romney said he would not give in to mounting attacks over his refusal to release his tax returns prior to 2010, including calls from some Republican allies to disclose the records and end the controversy.

"In the political environment that exists today, the opposition research of the Obama campaign is looking for anything they can use to distract from the failure of the president to reignite our economy," Romney told the conservative National Review Online.

"I'm simply not enthusiastic about giving them hundreds or thousands of more pages to pick through, distort and lie about," he told the magazine, which itself later published an editorial urging Romney to release more tax returns.

The Obama campaign put up a new television ad in the swing state of Pennsylvania raising questions about why Romney, the former head of a private equity firm, will not release more of his tax returns.

"Tax havens, offshore accounts, carried interest -- Mitt Romney has used every trick in the book," the ad's narrator said. "Romney admits that over the last two years he's paid less than 15 percent in taxes on $43 million in income. Makes you wonder if some years he paid any taxes at all," the ad said, concluding: "What is Mitt Romney hiding?"

On a conference call with reporters arranged by the Romney campaign, former New Hampshire Governor John Sununu called Obama and his campaign a "bunch of liars" and said: "I wish the president would learn how to be an American."

Obama, who has released a certificate showing he was born in Hawaii, still faces accusations from some conservatives that he was not born in the United States and should not be legally able to serve as president.

Romney has tried to avoid the so-called "birther" issue, and Sununu was quick to retreat from his statement. "What I thought I said but what I didn't say is the president has to learn the American formula for creating business," Sununu said.

He apologized in a later interview with CNN. "Frankly, I made a mistake," Sununu said.

'OFF THE DEEP END'

Sununu's comments showed Romney was desperate to change the conversation and his campaign had gone "off the deep end," Obama campaign spokeswoman Lis Smith said.

"The question is what else they'll pull to avoid answering serious questions about Romney's tenure at Bain Capital and investments in foreign tax havens and offshore accounts," she said.

The attacks highlighted the increasingly brutal tone of the presidential campaign. At a stop in Pennsylvania, Romney criticized Obama's economic leadership for failing to make a dent in the nation's 8.2 percent unemployment rate.

He opened a new line of attack on a comment last week by Obama suggesting that businesses are not just built by their owners. Romney said it was an example of Obama's hostile attitude toward free enterprise.

"It's insulting to every entrepreneur and every innovator in America," Romney said. "President Obama attacks success, and therefore under President Obama we have less success. And I will change that."

Obama has painted Romney, the former governor of Massachusetts, as too wealthy and out of touch to understand the economic difficulties of most Americans. His campaign has criticized Romney's leadership at Bain, questioning whether the company helped ship jobs overseas and if Romney kept an active role at the firm after saying he left in 1999.

At the first of four campaign stops in Texas on Tuesday, Obama said in San Antonio that Romney "made money investing in companies that have been called 'pioneers' in outsourcing. I don't want pioneers of outsourcing in the White House. I want somebody who believes in insourcing."

Some polls have shown the Obama attacks are taking a toll on Romney, although the two are still running close in most national polls ahead of the November 6 presidential election.

Prominent Republicans like former Mississippi Governor Haley Barbour, Alabama Governor Robert Bentley and conservative commentators George Will and Bill Kristol have called on him in recent days to release the returns.

Asked about Romney's taxes, Texas Governor Rick Perry said elected officials should be as open as possible.

"I'm a big believer that no matter who you are or what office you're running for, you should be as transparent as you can be with your tax returns and other aspects of your life so that people have the appropriate ability to judge your background," Perry, a former Republican presidential rival, told reporters in Austin.

While voluntary, presidential contenders have traditionally released their tax returns. Romney's father, George Romney, released 12 years of returns as part of his own failed 1968 presidential bid. Obama has released tax returns back to 2000.

Romney released his 2010 return and an estimate for his 2011 return in January at the height of the Republican presidential nominating fight.

The Obama campaign highlighted the growing Republican support for releasing the returns.

"This is a call that's not just being made by us, it's being made by many people, including people from Mitt Romney's own party," campaign spokeswoman Jen Psaki told reporters.

The Romney campaign also offered more signals on Tuesday that it might be nearing a choice on a running mate by naming two key members of the team that will support the eventual No. 2.

Speculation grew around a list of three: Ohio Senator Rob Portman, Louisiana Governor Bobby Jindal and former Minnesota Governor Tim Pawlenty. AGENCIES