In an online editorial, the conservative National Review urged Romney to release additional returns even though it agreed with him that the Obama campaign wanted them for a "fishing expedition."
Obama has been trying to keep Romney focused on matters other than the sluggish economy, even releasing a single-shot TV ad Tuesday that suggests that Romney gamed the system so well that he may not have paid any taxes at all for years.
As the campaign's tenor grew combative, Romney seized on comments Obama made while campaigning in Virginia last week. The president, making a point about the supportive role that government plays in building the nation, said in part: "Somebody helped to create this unbelievable American system that we have that allowed you to thrive. Somebody invested in roads and bridges. If you've got a business, you didn't build that. Somebody else made that happen."
Obama later added: "The point is, is that when we succeed, we succeed because of our individual initiative, but also because we do things together."
The challenger pounced.
"To say that Steve Jobs didn't build Apple, that Henry Ford didn't build Ford Motors, that Papa John didn't build Papa John Pizza ... To say something like that, it's not just foolishness," Romney said from a campaign rally outside Pittsburgh.
"It's insulting to every entrepreneur, every innovator in America."
Romney added: "I tell you this. I'm convinced that he wants Americans to be ashamed of success."
The Obama campaign said Romney had distorted Obama's message by taking it out of context. Obama's intended point - one he made again in Texas on Tuesday - was that government plays a role in helping people and businesses succeed by building roads, hiring teachers and firefighters, and looking out for the public good.
"There are some things we do better together," Obama said in San Antonio at the start of a lucrative fund-raising day in Texas. "We rise or fall as one nation. That's what I believe. That's what our history tells us. That's what our future demands. That's why I'm running for a second term as president of the United States."
A consistent part of Obama's "bottom-up" economic message is that individual initiative and hard work should be rewarded. But in a war of words, with both sides eager to jump on any gaffe or inelegant phrase, Romney saw a way to paint Obama as a big-government Democrat.
During the new line of attack, his campaign also questioned Obama's patriotism. Former New Hampshire Gov. John H. Sununu, a Romney supporter, told reporters that businesses that grow from the ground up made the U.S. economy the envy of the world.
"It is the American way, and I wish this president would learn how to be an American," he said.
Asked to clarify that incendiary remark, Sununu later said: "The president has to learn the American formula for creating business." Still later, he told CNN he had made a mistake. "I shouldn't have used those words. And I apologize for using those words."
Romney is trying shift attention away from his business record and his tax returns with a fresh assault as Obama, anxious about losing his fund-raising edge, turned to Republican-tilted Texas to raise millions of dollars from gay, Latino, and big-dollar donors.
During public events, Obama has remained focused on distinguishing himself from Romney's policies and background, limiting his criticism of Romney's private equity firm, Bain Capital, to claims that it promoted the outsourcing of jobs to countries like China and India.