Speaking to reporters in South Carolina, Mitt Romney said he had examined his tax returns for the past decade and that he had paid "at least 13 percent" of his income every year.

Earlier this month, Senator Harry Reid of Nevada, who leads the Democratic Party majority in the U.S. Senate, claimed that Romney had paid no taxes in some years, presumably by using tax shelters and other loopholes. Reid offered no proof to substantiate the allegation.

Romney on Thursday rejected Reid's claim.

"Harry Reid's charge is totally false," he said. "I'm sure waiting for Harry to put up who it was that told him what he says they told him. I don't believe it for a minute, by the way. But every year I've paid at least 13 percent, and if you add in addition the amount thatgoes to charity, why the number gets well above 20 percent."

In January, Romney released his full tax returns for 2010 and a summary of the taxes he paid in 2011. He has refused to release tax returns for other years.

The 2010 returns showed that he paid 13.9 percent of his income that year in federal taxes.

President Barack Obama, when he was running for the office in 2008, released eight years of tax returns, and has released his yearly returns while in office. They show that in 2010, Mr. Obama and his wife, Michelle, jointly paid an effective tax rate of just over 26 percent.

The Obamas earned $1.7 million in 2010, while Romney and his wife, Ann, earned more than $21.7 million that year. 

The top U.S. income tax rate is 35 percent.