Mitt Romney came under increasing pressure Wednesday to release his income tax returns immediately, rather than waiting until April, in an effort to contain the damage caused by his stumbling responses over two days.

Unanswered questions about his wealth threw the GOP presidential front-runner’s campaign off balance and threatened to undercut his message in the final days before Saturday’s potentially decisive primary in South Carolina, where a new poll showed Romney’s lead in the state shrinking to 10 percentage points.

Romney and his surrogates launched a fresh assault on rival Newt Gingrich on Wednesday, calling the former House speaker’s tenure in Washington “leadership by chaos” and likening him to former vice president Al Gore.

“It’s the private sector that creates jobs,” Romney said at a Spartanburg rally. “Congressmen taking credit for helping create jobs is like Al Gore taking credit for the Internet.”

But the offensive did little to quiet a growing debate among Republicans about Romney’s reluctance to release his tax returns.

Gingrich plans to release his own returns on Thursday and to tee up the tax issue for a debate Thursday night in Charleston. Former senator Rick Santorum (Pa.) said he will release his returns, too, while Texas Gov. Rick Perry said GOP voters need to vet Romney’s returns to determine whether he might “get eaten alive” against President Obama.

New Jersey Gov. Chris Christie, a Republican, was the most prominent Romney supporter to call publicly for the swift release of his returns, while Republicans close to the campaign said there was direct private pressure on the candidate and his Boston team to deal with the issue immediately.

“What I would say to Governor Romney is if you have tax returns to put out, you should put them out,” Christie said on NBC‘s “Today” show. “And you put them out sooner rather than later because it’s always better in my view to have full disclosure, especially when you’re the front-runner.”

The editors of National Review also called for swift release of the forms and pointedly described the former governor’s response to a question about the matter in Monday’s debate in Myrtle Beach “the weakest moment in his weakest debate.”

The editorial argued that, while there probably is nothing in the returns that would be terribly damaging, quick disclosure before Saturday’s primary would help reassure Republicans. “We know that should Romney become the nominee, he will be criticized over the sources of his wealth and will have to effectively respond,” the editorial said. “Republican primary voters deserve to see whether he can do so before they vote.”

The issue has dogged Romney since he first indicated that he might not release his returns. But after he shifted positions during a debate Monday and acknowledged Tuesday that he has paid an effective tax rate of about 15 percent, GOP strategists said the political cost of delaying the release has escalated dramatically.

“He looks guilty,” said Ed Rogers, who is unaligned. “It’s just going to be untenable for Romney to keep it up. . . . This new tax return issue is a new bumper sticker: ‘Release them.’ And a bumper sticker always beats an essay.”

Added Rogers, who co-writes an opinion blog at The Washington Post: “He’s got to own up to being rich, put it all out there and hope that it doesn’t bog down the campaign.”

Romney, a multimillionaire, committed a verbal gaffe Tuesday when he described the amount of money he had made from speeches as “not very much.” The sum was $374,000 last year — which alone would make him among the nation’s top 1 percent of earners.

Even Romney loyalists acknowledged Wednesday that the comment was politically damaging. They added to a list of verbal missteps by the former Massachusetts governor, who has looked to the South Carolina primary as a chance to effectively close down the GOP contest.

The handling of the tax issue has been uncharacteristic of the Romney campaign, which has repeatedly fended off problems — such as Perry’s widely celebrated entry to the race and Gingrich’s earlier rise in the polls.

“Why is he afraid to release [the returns]? Obviously, he’s got something to hide,” said Jim Woody, 59, a retired educator from Belton, S.C., who said he plans to vote for Gingrich. “It’s just like Obama — Romney and Obama. They just don’t want us to see what they did in the past.”

Romney aides said they were not caught off guard by the issue, saying Romney had planned to release his most recent tax return if he became the nominee.

“It would be presumptuous for anybody to put their tax returns out before they were the nominee of the party,” Romney communications director Gail Gitcho said.

She said there were no plans to release the documents sooner and would not say whether Romney would release past returns as well. She took issue with his rivals’s focus on the tax issue.

“If Newt Gingrich wants to talk about tax returns, we’re gonna talk about the economy,” Gitcho said. “They can talk about it all they want. Governor Romney’s going to talk about jobs and why we need a new direction in this country.”

Rivals are using the issue to try to blunt Romney’s momentum.

“It is important for Mitt to release his tax returns,” Perry said Wednesday on “Fox and Friends.” “The fact is, we can’t fire our nominee in September. By then, it’s too late. If we’ve got a flawed candidate going forward who’s going to get eaten alive either because of business practices or because of the taxes and the system that’s set up, we need to talk about it now.”

Staff writer Rosalind S. Helderman in South Carolina contributed to this report.