If Mitt Romney has nothing to hide, why is he acting like it?

Thursday the Republican presidential candidate said that he has never paid less than 13 percent of his income in taxes during the past decade. But he declined yet again to release the records to prove his statements.

"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," Romney told reporters after landing in South Carolina for a fundraiser.

Romney has released his 2010 tax return and has vowed to release his 2011 return before the Nov. 6 election, but has refused to release his returns from earlier years.

Citing an anonymous source, Democratic Senate Majority Leader Harry Reid, has charged that Romney didn't want to release any more of his returns because the Republican candidate had paid no taxes in some years.

"Harry Reid's charge is totally false. I'm sure waiting for Harry to put up who it was that told him what he says they told him," Romney said.

Obama campaign spokeswoman Lis Smith responded by telling Romney to "prove it."

"He has forfeited the right to have us take him just at his word," Smith said.

Romney commented as his campaign worked to stay on the offensive in the increasingly heated debate over the future of Medicare, the health care program relied upon by millions of seniors.

Paul Ryan's addition to the GOP ticket last weekend drew immediate scrutiny to a budget proposal he wrote that proposes to transform Medicare into a voucher-like system for future retirees.

In turn, Romney and Ryan called attention to President Barack Obama's health care law, which is funded in part by future savings from Medicare, and accused him of "raiding" the program of billions of dollars.

"What he probably did not mention yesterday is that when he passed his signature health care achievement, Obamacare, he raided $716 billion from Medicare to pay for Obamacare," Ryan, chairman of the House Budget Committee, said. "This will lead to fewer services for seniors."

However, what Ryan fails to mention is that his budget proposal includes those same savings, which are supposed to be realized through lower payments to hospitals and doctors, and by making the program more efficient.

Romney has said he would restore the Medicare cuts.

Romney was raising money in South Carolina.

Obama resumes campaigning Saturday with a pair of stops in New Hampshire.

Ryan is scheduled to campaign Saturday in Florida with his 80-year-old mother, Betty Douglas, a Medicare recipient and part-time resident, said spokesman Michael Steel.