WASHINGTON — White House hopeful Mitt Romney insisted Thursday he has paid at least 13 percent per year in taxes over the past decade, dismissing obsession over his personal finances as "small-minded."

Romney, a multimillionaire former investor, has been under assault by Democrats for his refusal to release pre-2010 tax returns and, while he kept his composure he sounded irked by a question about his taxes during a fundraising stop.

"I just have to say, given the challenges that America faces -- 23 million people out of work, Iran about to become nuclear, one out of six Americans in poverty -- the fascination with taxes I've paid I find to be very small-minded compared to the broad issues that we face," he told reporters at an airport near Greenville, South Carolina.

"But I did go back and look at my taxes, and over the past 10 years I never paid less than 13 percent. I think the most recent year is 13.6 or something like that. So I paid taxes every single year," he said.

"Every year I've paid at least 13 percent and if you add, in addition, the amount that goes to charity, why the number gets well above 20 percent."

Romney paid an effective tax rate of 13.9 percent in 2010, according to returns he has released, as his income from investments was taxed as a capital gain rather than under the higher rates applied for salaried income.

Those figures may not sit well with many middle-class Americans who pay a higher rate than Romney because they pay taxes on salaried income more than investment income.

The current highest US individual income tax rate is 35 percent.

Romney also slammed US Senate Majority Leader Harry Reid for charging earlier this month that Romney went 10 years without paying taxes.

"Harry Reid's charge is totally false," Romney 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."

Reid ignited a firestorm when he made his claim, which he said came from an an unnamed source who worked at Romney's former firm Bain Capital.

Romney has proposed cutting income tax rates by 20 percent, eliminating tax on investment income, getting rid of the estate tax and cutting the corporate tax rate.

Obama has said the Republican's tax proposal would hike taxes on families with children by $2,000 to pay for his $5 trillion tax plan which would mostly benefit the wealthy.