FLORENCE, South Carolina—Republican presidential front-runner Mitt Romney admitted on Tuesday that he pays an effective tax rate below that of many middle class families.

Asked by a reporter what his effective tax rate was, Romney replied: "It’s probably closer to the 15 percent rate than anything."

He continued: "For the past ten years, my income comes overwhelmingly from investments made in the past, rather than ordinary income or earned annual income. I got a little bit of income from my book, but I gave that all away. Then, I get speakers fees from time to time, but not very much."

A Congressional Research Service report published last year showed that some families making less than $100,000 pay an effective tax rate greater than 26.5 percent.

Romney has been dodging the question of his tax returns for years, refusing to release them in his ill-fated 2008 run. In his 1994 Senate bid, Romney made a campaign issue out of Sen. Ted Kennedy's refusal to release his tax returns.

Under attack last night from Texas Gov. Rick Perry, Romney said we "probably" would release them if he's the nominee in April.

Today Romney stopped equivocating, saying will release the returns in April if he is the nominee, adding most people would be interested in seeing how he's been doing in the past year.

His senior adviser Eric Fehrnstrom maintained last night that Romney would just be "following precedent" by releasing them in April.