Mitt Romney bowed to political pressure on Tuesday by promising to release his federal income tax returns, while estimating the rate he pays at about 15 percent and placing himself among the wealthiest Americans who earn most of their money from past investments.

Romney’s disclosure underscored the Republican presidential front-runner’s discomfort with talking about a key aspect of his biography — his money — and reignited the debate over whether his multimillionaire status makes it hard for him to relate to middle-class Americans.

Romney’s Democratic and Republican opponents alike pounced on his revelation by saying he has benefited from a basic unfairness in the tax structure. And as President Obama looked to Romney as his likely general-election opponent, the White House and its allies tried to stoke populist anger over income inequality by casting Romney as an out-of-touch multimillionaire.

Romney seemed to strike another clumsy note when he described his income from speakers’ fees — about $370,000 in a single year — as “not very much.”

Most Americans earn their income from wages and salaries, and the more they bring in, the higher their taxes are — up to 35 percent. Obama paid a rate of 26 percent on adjusted gross income of about $1.73 million in 2010, according to his tax return.

Most of Romney’s income, however, has come from capital gains, which include profits from the sale of stocks, bonds and real estate, and are taxed at a rate of 15 percent. Over the past 20 years, the wealthiest one-tenth of 1 percent of Americans have realized about half of all capital gains income — and received tax advantages criticized by billionaire Warren Buffett and Occupy Wall Street protesters alike.

Romney has resisted releasing his tax returns, but under renewed pressure, changed course. He said he would release his 2011 returns in April, but did not say whether he would release any earlier returns.

In discussing this one detail about his personal wealth, the former Massachusetts governor drew more unwanted attention to it — without putting the matter to rest. His remarks ballooned into a major news event on the campaign trail on Tuesday, threatening to overshadow his attempts to win over more South Carolina Republicans four days before the potentially decisive primary.

“What’s the effective rate I’ve been paying? It’s probably closer to the 15 percent rate than anything,” Romney said at a news conference here. “My last 10 years, I’ve — my income comes overwhelmingly from investments made in the past rather than ordinary income or rather than earned annual income.”

Romney, who has not drawn a salary for several years, said he received royalties from his 2010 book, “No Apology,” but added: “I gave that all away. And then I get speakers’ fees from time to time, but not very much.”

Romney collected more than $370,000 for such appearances, averaging more than $40,000 per speech from February 2010 to February 2011, according to a mandatory financial disclosure form filed last summer. Romney — a retired chief executive at the private equity firm Bain Capital — assessed his net worth as $190 million to $250 million.

Asked about Romney’s view that his speakers’ fees were “not very much,” New York Attorney General Eric Schneiderman said on a conference call organized by the Democratic National Committee: “I’ll bet you $10,000 the American people think it’s a lot.”

By referencing a $10,000 bet Romney tried to wager with Texas Gov. Rick Perry in a GOP debate, Schneiderman highlighted Romney’s awkwardness in dealing with his wealth.

In New Hampshire, the candidate said he worried about receiving a “pink slip.” In South Carolina, he handed an out-of-work supporter cash (his campaign said it was $50 to $60).

On Tuesday, Romney criticized a plan by former House speaker Newt Gingrich (Ga.), one of his rivals, to eliminate capital gains taxes for all Americans. “Warren Buffett, Bill Gates would probably pay no taxes at all,” Romney said, without stating whether it would affect him. Romney’s own plan eliminates capital gains taxes for those making $200,000 or less.

The Romney campaign did not respond to requests to discuss his taxes. But spokeswoman Andrea Saul distributed a National Review Online column she marked “MUST READ” by Douglas Holtz-Eakin, Sen. John McCain’s former economic policy adviser. He accused Democrats of demonizing Romney “for succeeding in the private sector.”

At 15 percent, Romney would pay a smaller share of his income in federal taxes than most high earners. In 2009, the top 1 percent of earners paid an average of 24 percent of adjusted gross income in federal taxes, according to Internal Revenue Service data. Romney’s rate is lower because virtually all of his income is from profits on investments, rather than earned wages.

Even at 15 percent, however, Romney would pay more than most Americans. IRS data show the average taxpayer forked over 11 percent of income in 2009. Under the country’s progressive tax system, the bottom 50 percent of earners paid less than 2 percent in income taxes, and millions of households had no income tax liability.

Separate payroll tax levies devoted to Social Security and Medicare add significantly to the average tax bill. But even with payroll taxes factored in, the average person earning $60,000 to $103,000 in 2011 will send only about 16.6 percent of their total earnings to Washington, according to the nonpartisan Tax Policy Center.

Former senator Rick Santorum (Pa.), another candidate for the GOP nomination, accused Romney of taking advantage of tax “loopholes.”

“I don’t know what my rate is, but I think I’d be really happy if it were 15 percent,” he told reporters. “Look, the tax code is broken. It’s not Governor Romney’s fault.”

White House press secretary Jay Carney said Obama thinks wealthy Americans who earn income from investments should not pay a lower effective tax rate than middle-income Americans.

“This only illuminates what he believes is an issue, which is that everybody who’s working hard ought to pay their fair share,” Carney said.

Perry, Santorum and Gingrich took particular issue with the April timing of Romney’s planned release, which could come after he has secured the GOP nomination.

“What is he saying to the people of South Carolina? ‘You’re not important enough for me to release my income tax, nor the people of Florida?’ ” Gingrich told reporters in Florence. “Either there is nothing there, so why isn’t he releasing it, or there is something there, so why is he hiding it?”

Montgomery reported from Washington. Staff writers Dan Balz, Rosalind S. Helderman and Karen Tumulty in South Carolina and Jia Lynn Yang in Washington contributed to this report.