U.S. Republican presidential candidate Mitt Romney has released his federal tax records, showing he earned over $42 million during the last two years and paid an estimated $6.2 million in taxes.

The records for Romney and his wife Ann, released by his campaign early Tuesday, put their tax rate at 13.9 percent in 2010 with an expected rate of 15.4 percent in 2011, far below the top tax rate of 35 percent for ordinary wage earners.

The records show most of Romney's income was earned on capital gains from various investments. The U.S. tax code places capital gains at a rate of 15 percent. His personal net worth is believed to be between $190 million and $250 million – wealth built during his career running a private investment firm.

Romney's decision to release his tax records comes in the aftermath of his upset loss in last week's South Carolina primary election to former House speaker Newt Gingrich, who attacked Romney for his reluctance to release the records.

Romney's vast fortune will likely place the former Massachusetts governor in the middle of the growing debate over income inequality in America. Critics say his private investment firm bought dozens of businesses and laid off the employees, while making a profit for himself and his investors.

The Romneys also made about $7 million in charitable contributions during the last two years, much of it to the Mormon Church, of which he is a prominent member.

Their returns also show that some of his financial holdings are being held overseas in places such as the Cayman Islands, where many international investors place their income to avoid paying U.S. taxes. Romney's advisors say the overseas holdings have been reported on tax returns.