White House hopeful Mitt Romney offered his fullest explanation yet about his tax status Thursday, saying he paid at least 13 percent a year over the past decade, but President Barack Obama’s campaign offered a prompt retort: prove it.

Romney, a multimillionaire former investor, has been under assault by Democrats for refusing to release pre-2010 tax returns, and he appeared irked by the question about his personal finances in the midst of some of the most heated rhetoric of the 2012 campaign.

“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.