Washington (CNN) -- Republican presidential hopeful Mitt Romney said Sunday he will release his 2010 tax return and an estimate of his 2011 tax liability on Tuesday.
The comment, made in an interview on "Fox News Sunday," was a big change in Romney's plans for handling an issue that dogged his campaign last week, and followed Saturday's loss to former House Speaker Newt Gingrich in the South Carolina primary.
Last week, Romney told reporters he would release his tax returns in April and estimated his actual tax rate was close to 15% -- the amount charged for capital gains income -- because most of his income was from investments.
On Sunday, he acknowledged that strategy didn't work in response to reporters and rival candidates questioning when the former Massachusetts governor -- a multimillionaire -- would make public his tax details.
"I think we just made a mistake in holding off as long as we did. It was just a distraction," Romney said.
"I was planning on releasing them in April when they've been released by other candidates in the past," he said. "But you know what, given all the attention that has been focused on tax returns, given the distraction that I think they became in the last couple of weeks ... I will release my tax returns for 2010, which is the last returns that were completed."
Romney added that the tax return, as well as the 2011 estimate, would be posted on the campaign website Tuesday so that "people can take a good look at it."
Gingrich, who has raised the issue repeatedly, commended Romney when told during an interview on NBC's "Meet the Press" of the announcement.
"I think it's exactly the right thing to do," Gingrich said. "And as far as I'm concerned, that particular issue is now set aside and we can go on and talk about other, bigger, and more important things."
Democrats also have called for Romney to release his tax returns, hoping it will bolster their efforts to portray him as a wealthy financier out of touch with working Americans.
By succumbing to the growing pressure to release tax return details now, Romney acknowledged the issue hurt his once-soaring campaign as the nominating process shifts to the Florida primary on January 31.
A race that seemed headed toward a quick nomination for Romney went through a volatile week that changed the political equation for the four remaining Republican candidates vying to run against President Barack Obama in November.
Initially announced the victor in the January 3 Iowa caucuses by eight votes over former Pennsylvania Sen. Rick Santorum, Romney learned Thursday that certified results showed Santorum actually won the state by 34 votes.
Then came Gingrich's solid victory Saturday in South Carolina in a race that Romney had led until his uncertain handling of the tax return issue on the campaign trail and in Thursday's debate.
Asked at the debate by moderator John King of CNN if he would follow his father's example and release tax returns for multiple years, Romney answered "maybe" and appeared uncomfortable discussing the issue all week.
Two of South Carolina's longest-serving elected officials attributed campaign mistakes by Romney for helping Gingrich run away with the state's first-in-the-South primary.
Republican Sen. Jim DeMint, a powerhouse in conservative politics, said on CNN's "State of the Union" that Romney bungled the tax return issue because "he wasn't definitive in his answer."
"He should have said immediately 'I'll do it in April,' or 'I'm not going to do it,' or 'I'm going to do it immediately.' But the fact that he wasn't clear, I think, is what hurt him," said DeMint, who endorsed Romney in 2008 but has yet to declare his support for anyone this time.
Democratic Rep. James Clyburn, who serves as the assistant minority leader in the House of Representatives, pinned Romney's loss on an inability to connect with South Carolina Republicans.
"Romney seemed not to be able to connect with his base; he really separated himself from the voters," Clyburn said. "It was very clear to me that he was cutting himself off. ... He was not doing well with identifying with ordinary voters. He doesn't seem to be able to do that."
Santorum, also on CNN, said the Republican race so far -- with three different candidates winning each of the first three states -- dealt a serious blow to any notion that Romney was certain to emerge as the Republican presidential nominee.
In particular, he said the victories by both himself and Gingrich showed that conservatives were flexing their muscles in the primary process.
"This idea that Mitt Romney is not going to be able to be defeated unless conservatives coalesce, well, it's objectively false," Santorum said.
Romney, meanwhile, aimed more fire at Gingrich -- portraying him as a Washington insider who lacked the necessary experience outside the U.S. capital to fix the government's problems.
"I don't think that the people of this country are going to choose as the next president of the United States a person who spent 40 years in Washington as a congressman and a lobbyist," Romney said on Fox, adding that Gingrich "is not ideally suited to face off against the president."
He said that Gingrich proved to be a "failed leader" during his four years in the 1990s as speaker of the U.S. House of Representatives, claiming he resigned "in disgrace." Romney noted that a vast majority of GOP representatives joined Democrats in reprimanding Gingrich after he was ordered to pay $300,000 following an ethics probe.
"He has not had a record of successful leadership," Romney said during a rally Sunday in Ormond Beach, Florida.
Gingrich gave his explanation for what happened with that ethics probe on CNN's "State of the Union." He characterized the penalty leveled by the House Ethics Committee as reimbursement for the cost of the investigation.
He also claimed that he persuaded fellow Republicans in the House of Representatives to vote "yes" on the ethics charges against him in order to put a swift end to the proceedings. Doing so helped lawmakers move on the balancing the budget, Gingrich said.
"I personally asked House Republicans to vote 'yes' because we had to get it behind us to get back to the things that mattered," Gingrich said.
According to the nonpartisan fact check group PolitiFact, Gingrich was reprimanded by the House and ordered to pay the $300,000 penalty in 1997 for violating an ethics rule. It noted the charge against Gingrich was an ethics violation, though the $300,000 penalty was considered reimbursement for the ethics committee's investigation.
The violation originated in a course he taught at Kennesaw State College, which organizers claimed qualified for tax-exempt status, PolitiFact reported. The House Ethics Committee ultimately concluded the course was run to "help in achieving a partisan, political goal," making it ineligible for tax exemption, according to PolitiFact.
Central to the 1997 investigation was a letter submitted by Gingrich's lawyers, which the ethics panel deemed inaccurate. Gingrich conceded Sunday the letter was a mistake.
"It was a mistake," Gingrich said Sunday. "So the one mistake I made was signing a letter written by our lawyers, a firm which frankly did me a great disservice. And that's the only thing."
In another interview, Gingrich told C-SPAN's "Washington Journal" he planned a week of big speeches offering "big solutions for a big country" in the run-up to the Florida primary.
"I'll be at the space coast in Florida this week giving a speech -- a visionary speech -- on the United States going back into space in the John F. Kennedy tradition," Gingrich said, promising other speeches in following days on health care, housing, the economy, Cuba and Latin America.
Also taking to podiums this week will be Obama, who delivers his annual State of the Union address on Tuesday. Romney has announced plans for both a "prebuttal" and rebuttal to the president's speech.
On Sunday, Gingrich repeated his long-standing accusation that Romney has shifted his stance on major issues such as abortion throughout his career, which Gingrich said raised questions about Romney's true beliefs and viability as a presidential candidate.
"It does show you why he keeps bouncing around trying to find a message," Gingrich said on the CBS program "Face the Nation."
Romney hit back Sunday, saying that he would make Gingrich's character an issue in the campaign. Along with the congressional ethics violation, Gingrich has been married three times, twice divorcing to wed someone with whom he was having an affair.
"Character is a big part of leadership, as is vision, sobriety, steadiness," Romney said. "These are attributes which I think people want to see in their candidate. That's one reason we go through this grueling process of selection of our nominees. We get a good chance to really see them under fire, see how they respond."
As to Santorum, he tried to brush off his third-place showing in South Carolina during a rally Sunday in Coral Springs, Florida, saying Gingrich's roots in nearby Georgia gave him a distinct advantage.
Making no mention of Texas Rep. Ron Paul, Santorum touted himself as a "conservative firebrand" who he had the best chance of appealing to moderate, blue-collar voters in key swing states like Florida.
The former Pennsylvania senator continued his fiery rhetoric regarding Iran, saying he may tell that Middle Eastern nation's leaders "we will dismantle" their nuclear program if he becomes president. He argued that, if Iran develops nuclear weapons, it would have "carte blanche to spread a reign of terror around not just the Middle East, but ... western civilization."
"They're just as radical as the people who run al Qaeda -- their theology is identical," Santorum said of Iran's leaders. "We cannot allow the equivalent of al Qaeda to have this weapon."
CNN's Kevin Liptak and Gregory Wallace contributed to this report.