WASHINGTON — Mitt Romney said Thursday that he paid at least 13 percent of his income in taxes each year during the past decade, again confronting an issue that Democrats have used to portray him as out of touch with middle-class people.

Calling the interest in his personal tax returns "small-minded" in light of the nation's problems, Romney said he had nonetheless examined the last 10 years of his personal tax returns after Democrats suggested he might not have paid anything at all in some years.

"Every year, I've paid at least 13 percent," he said, referring to his effective federal income-tax rate, which is a higher effective rate than most people pay.

Romney's decision to address the tax question Thursday appeared to be an off-the-cuff attempt to put the issue behind him.

But at least initially, it had the opposite effect. Democrats seized on his comments to again demand proof of his claims by releasing multiple years of his tax returns.

"We would say: 'Prove it, Governor Romney,' " said Ben LaBolt, a spokesman for President Obama.

The re-emergence of the tax issue consumed another day of the campaign and added to the sense of a shift in direction for a candidate who had once steadfastly refused to talk about anything other than job losses during Obama's tenure, the unemployment rate and the nation's growing debt.

After Romney's decision to name Rep. Paul Ryan of Wisconsin as his vice-presidential choice, the campaign is instead waging an aggressive battle on Medicare, welfare and Obama's character. That change in focus can be seen in the campaign's ads and in Romney's speeches.

Romney made the remarks about his taxes Thursday at a hastily arranged news conference at an airport in South Carolina, where he sought to amplify the Medicare argument he had been making since selecting Ryan. To drive home his point, Romney drew numbers on a white board.

It was a moment that delighted aides back at his campaign headquarters in Boston — until Romney answered a question about his income taxes. For the rest of the day, his statement about his taxes received more attention than his original statement on Medicare.

Sen. Harry Reid of Nevada, the Democratic leader, has asserted — without providing proof — that Romney paid no taxes in some years, presumably by using offshore tax shelters and other legal accounting measures.

Romney had already denounced the remarks as false. But he has refused to release more than his full return for the 2010 tax year and a short summary of taxes he paid in 2011.

In saying that he paid a tax rate of at least 13 percent, Romney and his wife would still have had a higher income-tax rate than most households.

More than 46 percent of households did not pay any federal income tax in 2011 because their income was low enough that deductions and credits reduced their bill to zero.

Even a typical household making $100,000 a year would pay closer to a 10 percent average federal income tax rate than a 15 percent rate, Congressional Budget office data suggest.

In 2011, Obama and his wife reported an effective federal income-tax rate of 20.5 percent. In 2010, their rate was just more than 26 percent.