|Republican presidential candidate Mitt Romney, left, and Newt Gingrich debate in Tampa [Reuters]
Under mounting pressure from rivals, US Republican White House candidate Mitt Romney has released two large sets of tax documents disclosing his multi-million-dollar income on his campaign website.
The figures show Romney paid $3m in 2010 for an effective tax rate of 13.9 per cent, and estimates he will pay $3.2m for his income last year, a rate of 15.4 per cent.
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The documents revealed the former venture capitalist, who is among the richest individuals to run for president, had a reported income of $21.7m in 2010 and $20.9m last year.
The Romney family has also given away $7m in charitable contributions since 2010 - primarily to the Mormon Church.
His family paid $6.2m in federal income taxes during the same period, the documents showed.
The documents showed virtually all of Romney's income is derived from investments.
Under US tax law, income from investments is usually taxed at 15 per cent, while wages, the main source of income for a majority of Americans, can be taxed at up to 35 per cent.
Buckles to pressure
Rival candidates had been hounding Romney, a former Massachusetts governor, to release his tax returns in line with other presidential candidates past and present.
At a debate last week, Romney stood by his earnings, saying: "I know the Democrats want to go after the fact that I've been successful. I'm not going to apologise for being successful."
Romney's opponents, both Democrats and Republicans, have accused him of preying on companies during his years at the head of equity firm Bain Capital, reaping huge profits while gutting firms and erasing jobs.
Romney had hinted that he would release his returns after the April filing deadline for 2011, but appears to have changed his mind after his crushing loss to Gingrich in Saturday's South Carolina primary.
Obama and his Democratic Party have argued that millionaires and big corporations should pay more in taxes to help trim the country's massive debt, while Republicans oppose any tax raise, saying it would stunt job creation.
All taxes paid
A campaign spokesman said Romney had paid all the taxes required on all of his income, both domestic and overseas.
The nearly 550 pages of tax documents detail far-flung holdings in tax havens such as Luxembourg, Ireland and the Cayman Islands - but none of the offshore accounts showed much income.
Romney has said his business acumen makes him the candidate most capable of steering the country out of its economic downturn.
He said that he has turned around several companies and generated tens of thousands of jobs while at Bain.
Romney and Gingrich are locked in an bitter struggle over delegate-rich Florida, the next state to vote in the months-long process to select a Republican to challenge Obama in November.
The country's growing economic inequality promises are to be a major issue in the general election, with Republicans blaming Obama for the sluggish economic recovery and the president accusing his rivals of ignoring the struggling middle class.