U.S. Republican presidential candidate Mitt Romney earned over $42 million during the last two years, according to federal tax forms released by his campaign Tuesday.

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.

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

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

Romney and his wife, Ann, paid about $3 million in taxes in 2010, and will pay an estimated $3.2 million on his 2011 returns, putting their effective tax rate at under 15 percent, far below the top tax rate of 35 percent for ordinary wage earners. The U.S. tax code places capital gains at a rate of 15 percent.

The Romneys also made about $7 million in charitable contributions during that period, 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.