Tax returns for U.S. Republican presidential nominee Mitt Romney and his wife Ann show the couple had $13.7 million in income last year.

The returns, released by his campaign Friday, showed they paid more than $1.9 million in federal taxes, an effective tax rate of 14.1 percent. That is a lower rate than many Americans pay. But most of the Romneys' income was earned through investments, which in the U.S. are taxed at a lower rate than the wages of workers.

Earlier in the year, the Romneys released their 2010 return showing $21.6 million in income. They paid about $3 million in taxes two years ago.

Romney, a one-time venture capitalist, is one of the wealthiest-ever U.S. presidential candidates. Release of his tax returns has been a contentious issue as he seeks to oust the Democratic incumbent, President Barack Obama.

Democrats supporting Mr. Obama's re-election have called on the Republican challenger to release multiple years of his tax returns, as several U.S. presidential contenders have done over the years, including Mr. Romney's father George in the 1960s. But Mr. Romney has repeatedly declined.

His campaign said Friday that over a 20-year period ending in 2009, the Romneys paid an effective annual rate of 20.2 percent.

Mr. Obama and his wife Michelle released their 2011 return last spring, showing nearly $800,000 in income. They paid more than $162,000, an effective tax rate of 20.5 percent.

The two candidates campaigned Friday in key battleground states, with the president holding a rally in a Virginia suburb just outside the capital, Washington. Mr. Romney headed to the western state of Nevada.

Public opinion polls show Mr. Obama leading Mr. Romney in Virginia and most of the other so-called swing states expected to decide the November 6 election. The closely fought battleground states are important because U.S. presidential elections are essentially a collection of state-by-state contests, with each state's influence on the outcome weighted roughly by the size of its population.