(AP) - Mitt Romney, one of the wealthiest candidates ever to seek the presidency, paid nearly $2 million in federal taxes on $13.7 million in income that he and his wife reported last year, his U.S. returns showed Friday. That came to an effective tax rate of 14.1 percent, lower than millions of middle-income Americans but actually more than he had to pay.

Most of Romney's income was from investment returns. That is why his rate was lower than taxpayers whose income was mostly from wages, which can be taxed at higher rates.

Romney's taxes have emerged as a key issue during the 2012 presidential race with President Barack Obama. Romney released his 2010 returns in January, but he continues to decline to disclose returns from previous years - including those while he worked at Bain Capital, the private equity firm he co-founded.

The Obama campaign and other Democrats have pushed for fuller disclosures, reminding the Republican candidate that his father, George Romney, released a dozen years of returns when he ran for president.

Overall, the Romneys' main tax return and separate forms for blind trusts totaled over 800 pages. The blind-trust income came from hedge funds and other complex investment vehicles. The couple also reported $3.5 million in income "from sources outside the United States," citing "various countries." Their forms included filings on holdings in Switzerland, Ireland, Germany and the Cayman Islands.

The Obama campaign accused Romney anew of profiting from millions invested overseas and "loopholes and tax shelters only available to those at the top."

Apparently hoping to resolve basic questions voters might have, the Romney campaign also released a letter from his accountants saying that in the 20 years prior to 2010 the Romneys paid an average annual effective rate of 20.2 percent, never lower than 13.66 percent. On average, middle-income families - those making from $50,000 to $75,000 a year - pay 12.8 percent of their income in federal taxes, according to Congress' Joint Committee on Taxation. But many pay a higher rate.

The former Massachusetts governor, whose wealth is estimated at perhaps $250 million, is aggressively competing with Obama for the support of middle class voters.

Obama's own tax return for last year showed that he and his wife, Michelle, paid $162,074 in federal taxes on $789,674 in adjusted gross income, an effective tax rate of 20.5 percent. Their income plunged from $1.7 million in 2010, with declining sales of the president's books. In 2009, the Obamas reported income of $5.5 million, fueled by the best-selling books.

The Romneys' tax bill could have been lower.

For the year, they claimed a deduction for $2.25 million of their $4.021 million in charitable contributions, said Brad Malt, trustee of the candidate's blind trust.

The Romneys gave $2.6 million in cash to the Church of Jesus Christ of Latter-day Saints, the documents show. They gave just over $2 million in non-cash charitable contributions - including donations of stock holdings in Domino's Pizza, Dunkin Donuts and Warner Chilcott - to a family trust.