WASHINGTON — Mitt Romney’s newly released tax returns represent an extraordinary accounting of the household finances and far-reaching corporate investments of one of the richest U.S. presidential candidates in generations, with an annual income that tops $20 million.

How the details of Romney’s extensive wealth will play among Republican taxpayers, rival campaigns, the media and the American public only started to emerge Tuesday, as more than 500 pages from a 2010 tax return and a 2011 estimate spilled out both significant and minor revelations about Romney’s scattered holdings, tax strategies and charitable donations.

The returns outline both the dimensions of Romney’s finances and the complexity of the tactics used to reduce his effective tax rate close to the low 15 percent paid by many middle-class Americans. Among the new details contained in the documents are Romney’s continuing profits from the private equity firm he founded but no longer runs, a Swiss bank account closed just as Romney launched his White House run and new listings of investment funds that were set up in offshore locations from the Caribbean to Ireland and Luxembourg.

Romney’s advisers stressed that he met all his federal tax obligations, provided maximum transparency and did not take advantage of “aggressive” strategies often used by the ultra-rich. Still, for millions of American taxpayers who are just beginning to grapple with their latest returns as tax season looms, Romney’s multimillion-dollar returns provide a window into an unfamiliar world.

Tax law experts familiar with the formidable financial portfolios of investment fund managers said Romney’s returns would at the very least reinforce the rising public issue of income inequity.

“The average American has a hard time understanding their own two-page tax return let alone Gov. Romney’s 200-page return,” said Joseph Bankman, a Stanford University professor of business and law who has testified to Congress on tax issues. “What would jump out at anyone is the sheer amount of money and low tax rate he pays, as well as the enormous complexity of his financial transactions.”

Romney paid about $3 million in federal income taxes in 2010, having earned more than seven times that from his investments. That income, $21.7 million, put him among the wealthiest of American taxpayers. Romney’s campaign said Tuesday he followed all tax laws.

At the same time, Romney gave nearly $3 million to charity — about half of that amount to the Mormon Church — which helped lower his effective tax rate to a modest 14 percent, according to records his campaign released Tuesday.

Romney’s income puts him in the top 0.006 percent of Americans, based on the most recent Internal Revenue Service data, from 2009. That year, only 8,274 filers reported income above $10 million.

He could be worth up to $250 million, based on previously released financial information.

The documents were released as President Barack Obama prepared to deliver his State of the Union message, in which he is expected to talk about economic fairness.

Asked during a round of TV interviews Tuesday about Romney’s tax rate, given that he’s a multimillionaire, White House adviser David Plouffe said: “We need to change our tax system. We need to change our tax code so that everybody is doing their fair share.”

Other Democratic Party voices were less restrained. “He used every loophole in the book available to the wealthy and corporations to avoid paying his fair share,” said Democratic National Committee Executive Director Patrick Gaspard.

Romney’s GOP rivals had no immediate comment. But House Speaker John Boehner, R-Ohio, defended the Romney’s tax rate as being close to what most Americans pay on long-term capital gains from the sale of investments.

“We all know that there’s a reason we have low rates on capital gains,” Boehner told reporters “That is because it spurs new investment in our economy and allows capital to move more quickly.”

Romney had long refused to disclose any federal tax returns, then hinted he would only offer a single year’s return in April. But mounting criticism from his rivals and a hard loss in last week’s South Carolina primary forced his hand.

For 2011, Romney will pay about $3.2 million with an effective tax rate of about 15.4 percent, the campaign said. Those returns haven’t yet been filed yet.

In total, he would pay more than $6.2 million in taxes on $45 million in income in the past two years, his campaign said.

“Gov. Romney has paid 100 percent of what he owes,” said Benjamin Ginsberg, the Romney campaign’s legal counsel. Ginsberg and other advisers insisted Romney did not use any aggressive tax strategies to help reduce or defer his tax income.

The advisers acknowledged that Romney continues to earn money from investments from Bain Capital, the Boston-based private equity firm the candidate founded and managed between 1984 and early 1999. Under an agreement with the firm when he left, Romney continued to earn “carried interest” on new Bain investments as a former partner in the firm even though he no longer ran the operation.

Romney earned $7.5 million in Bain earnings in 2010 and expects to make $5.5 million in 2010, Ginsberg said.

The former Massachusetts governor had been cast by his GOP opponents as a wealthy businessman who earned lucrative payouts from his investments while Bain slashed jobs in the private sector. Rival Newt Gingrich released his 2010 returns last Thursday showing he paid almost $1 million in income taxes, a tax rate of about 31 percent.

Romney’s advisers acknowledged Tuesday that Romney and his wife, Ann, had a bank account in Switzerland as part of her trust. The account was worth $3 million and was held in the United Bank of Switzerland, said R. Bradford Malt, a Boston lawyer who makes investments for the Romneys and oversees their blind trust, which was set up to avoid any conflicts of interest in investments during his run for the presidency.

In 2009, UBS admitted assisting U.S. citizens in evading taxes, and agreed to pay a $780 million penalty as part of a deferred prosecution agreement with the U.S. Justice Department.

Malt said he closed the account in early 2010 for “diversification” and because it “just wasn’t worth it.” He sidestepped a question about whether he did so because the account could have been a political liability, saying it “might or might not be inconsistent with Gov. Romney’s political views.” Malt has sold off other accounts in recent years — including investments in firms that did business with Iran and China — because of possible political inconsistency or embarrassment with Romney’s political positions.

Malt also confirmed that some of Romney’s investments are routed through affiliate funds set up in the Cayman Islands. He insisted there were no actual offshore accounts, and added that Romney paid the same amount of U.S. taxes using the Cayman affiliates as he would have if the investment funds were set up in the U.S.

Romney’s 2010 tax return also shows a number of foreign investments, including funds based in Ireland, Switzerland, Germany and Luxembourg. The documents also detailed another investment fund routed through a Bain Capital affiliate set up in Bermuda.

The returns showed about $4.5 million in itemized deductions, including $1.5 million to the Church of Jesus Christ of Latter-Day Saints.

Romney’s charitable giving is above average, even for someone at his income level. In 2009, more than 37 million filers claimed charitable deductions averaging more than $4,000. Among those making more than $10 million, the average charitable deduction was about $1.7 million, according to the IRS.

Before the tax records were released, Romney’s old investments in two government-backed housing lenders stirred up new questions at the same time his campaign targeted Gingrich for his work for Freddie Mac.

Gingrich earned $1.6 million in consulting fees from Freddie Mac. Romney has as much as $500,000 invested in the U.S.-backed lender and its sister entity, Fannie Mae.

The fight over releasing the tax information highlighted an argument that Democrats are already starting to use against Romney — that he is out-of-touch with normal Americans. And it probably hurt him in the South Carolina primary, where he lost by 12 percentage points to Gingrich after spending several days resisting calls to release the returns.

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Associated Press Kasie Hunt reported from Tampa and Stephen Ohlemacher from Washington.