For the often-reserved Romney, the fresh attacks marked a substantial escalation in aggression for a candidate who has struggled to answer questions about his business career and personal tax returns. The former businessman, who would be among the nation's wealthiest presidents if elected, has so far released just one year of personal income tax returns and promised to release a second.
That's a stark deviation from a tradition created in part by Romney's father, George, a presidential candidate a generation ago who released 12 years of his returns.
A defiant Romney has accused the Obama campaign of using the issue to distract voters from his handling of the economy less than four months before the Nov. 6 election.
But it's unclear if Romney's new strategy will be enough to change the subject. Several prominent Republicans joined Democrats in pushing Romney for more transparency.
Texas Gov. Rick Perry — who challenged Romney for the GOP nomination — became the latest top conservative to pressure Romney to open his finances. Perry, who has released his tax returns dating back to 1992, said anyone running for office should make public as much personal information as possible to help voters decide.
The conservative National Review also urged Romney to release more tax returns even though it agreed with him that Obama's camp wanted them for a "fishing expedition."
"By drawing out the argument over the returns, Romney is playing into the president's hands," the magazine said in an online editorial. "He should release them, respond to any attacks they bring, and move on."
Romney spokesman Kevin Madden said Romney will not bow to the pressure.
"The governor has gone above and beyond what's required for disclosure," Madden said. "The situation remains the same."
Former Minnesota Gov. Tim Pawlenty, who features prominently in speculation about Romney's choice for a running mate, vigorously defended Romney's limited tax release.
"There is no claim or no credible indication that he's done anything wrong," Pawlenty said Wednesday on "CBS This Morning."
Pawlenty accused Obama's campaign of "hanging shiny objects before the public and the press, and the press is taking the bait."
Romney's wife, Ann, meanwhile, shed some light Wednesday on her husband's vice presidential search. A top aide to the candidate earlier had suggested that an announcement could have come by the end of the week.
In an interview with ABC News, scheduled for broadcast Thursday, Ann Romney said her husband had yet to settle on a candidate.
"We're not quite there yet," she said, according to excerpts released by the network.
Separately, Romney's campaign was forced to apologize after a supporter questioned Obama's patriotism.
In a conference call Tuesday arranged by the campaign, former New Hampshire Gov. John H. Sununu told reporters he wished Obama "would learn how to be an American." He later apologized.
"I made a mistake. I shouldn't have used those words. And I apologize for using those words," Sununu told CNN. "But I don't apologize for the idea that this president has demonstrated that he does not understand how jobs are created in America."
- Supreme Court Justice Antonin Scalia is dead...
- Fight night: Personal attacks, court debate...
- In victory or dissent, Scalia was a man of...
- Patient strategy pays off for FBI in ending...
- See why some are calling Sports Illustrated's...
- Clinton, Sanders clash over minorities, money...
- Republican group likens Clinton to Trump in...
- IMAX film 'National Parks Adventure'...
- Obama sends Congress record $4.1... 29
- Clinton, Sanders clash over minorities,... 28
- Trump, Sanders victorious in New... 27
- Supreme Court Justice Antonin Scalia is... 26
- Exit polls: Dems trust Sanders more... 23
- But is she honest? Caring? Clinton... 23
- FBI says it has surrounded last... 23
- Last occupiers of Oregon wildlife... 19