Republican presidential candidate Mitt Romney may have finally revealed the reason he has refused to release his tax returns: Because he pays the federal government a much lower rate than many middle-class Americans.

Speaking to reporters in Florence, South Carolina on Tuesday, the multimillionaire candidate explained that he pays an effective rate of only around 15 percent.

"It’s probably closer to the 15 percent rate than anything," Romney said. "Because my last 10 years, I’ve — my income comes overwhelmingly from some investments made in the past, whether ordinary income or earned annually. I got a little bit of income from my book, but I gave that all away."

"And then I get speaker's fees from time to time, but not very much," he added.

As The New York Times noted, the former Massachusetts governor made $374,327.62 in speaker's fees in the most recent year. On average, he made $41,592 per speech, something that most Americans would not describe as "not very much."

A report from the nonpartisan Congressional Research Service last year found that 10.4 million moderate-income taxpayers paid more than 26.5 percent in taxes.

During a Fox News debate in Myrtle Beach, South Carolina Monday, Romney made several attempts to dodge questions about whether he would ever release his tax returns.

"You know, if that’s been the tradition and I’m not opposed to doing that, time will tell," the candidate said. "But I anticipate that most likely I am going to get asked to do that around the April time period and I’ll keep that open."

"I sort of feel like we are showing a lot of exposure at this point. And if I become our nominee, and what's happened in history is people have released them in about April of the coming year and that's probably what I would do."

Watch this video from CNN, uploaded by Think Progress on Jan. 17, 2012.