(AP) - Mitt Romney, one of the wealthiest candidates ever to seek the presidency, paid nearly $2 million in federal taxes on the $13.7 million in income reported for himself and his wife last year, his U.S. returns showed Friday. That comes 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.

There also has been Democratic criticism of Romney's foreign investments. Several tax law experts said Friday that his newly released tax returns would not be much help in resolving critics' questions about his sprawling finances - whether he used aggressive tax-deferral strategies, what might be the specifics and tax advantages of his numerous offshore investments, what was the source of his massive retirement account and what are the details behind his now-closed $3 million Swiss bank account.

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 President Obama for the support of middle class voters.