FLORENCE, S.C. – Seeking to protect his standing here, Republican presidential front-runner Mitt Romney on Tuesday assailed challenger Rick Santorum’s record on voting rights for felons, an issue that resonates strongly with conservatives. Santorum accused the former Massachusetts governor of promoting lies about him as the two intensified their dispute from the previous night’s GOP debate.

Romney tried to tamp down another contentious, lingering issue by offering a snippet of his personal tax picture while saying he wouldn’t publicly release his tax return until April. The multimillionaire former businessman said he pays an income tax rate close to 15 percent – well below the 35 percent applied to the nation’s top earners – because his income mostly comes from investments.

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Presidential candidate Mitt Romney speaks to reporters after a campaign appearance Tuesday at the Florence Civic Center in Florence, S.C. The Republican front-runner’s rivals are pushing Romney to release his tax returns now. The Associated Press

Romney, hoping a strong and third straight win in Saturday’s South Carolina primary will all but lock up the nomination, made a point Tuesday of answering criticism from Santorum, the former Pennsylvania senator trying to unite conservative voters to block Romney’s path.

“We have a candidate who’s not going to stand up and tell the truth,” Santorum told reporters in Charleston, complaining that Romney refuses to condemn misleading TV ads. “That leads to real serious questions about whether that man can be trusted to tell the truth on a variety of things.”

Santorum was particularly upset about an ad that says he “even voted to let convicted felons vote.” Santorum complained that the TV spot, while referring to “felons,” shows someone in an orange prison jumpsuit, suggesting Santorum would allow them to vote while still incarcerated. He has supported voting rights only for those who have served their sentences and been released.

But Romney noted that “people who have been released from prison are still called felons if they’ve committed felonies.”

In Florence, Romney defended the accuracy of the ads in question, although he maintains he has no control over the negative commercials that are flooding South Carolina media by outside groups that support his candidacy.

“I hear that Rick Santorum is very animated that the super PAC ad says that he is very in favor of felons voting,” Romney said. “Well, he is! That’s his position.”

Romney’s other three challengers tried a host of different attacks in hopes of knocking him off stride in the debate. They kept the spotlight on his wealth and business dealings by pressing him to release his income tax returns before this weekend’s vote.

Romney told reporters Tuesday that he would release his 2011 tax return in April.

“What’s the effective rate I’ve been paying? It’s probably closer to the 15 percent rate than anything,” Romney said, referring to the tax rate on investment income. The average American household will pay 9.3 percent in federal income taxes, although that rate climbs to 19.7 percent when other federal payroll taxes, for Social Security and Medicare, are included. Investment income isn’t subject to those taxes.

Former House Speaker Newt Gingrich suggested Romney wouldn’t delay releasing his tax information if he had nothing to hide, and that the delay wouldn’t sit well with voters in this month’s primaries. “What is he saying to the people of South Carolina?” Gingrich said Tuesday. “You’re not important enough for me to release my taxes? Nor are the people of Florida?”

Romney also came under heavy pressure in the debate on the issue of his job-creation record at his former private equity firm Bain Capital, and his evolving views on abortion.

Meanwhile, Gingrich picked up the support of South Carolina Lt. Gov. Ken Ard, who appeared with him in Florence, calling the former U.S. House speaker the smartest and toughest candidate. Romney already has the more coveted endorsement of Gov. Nikki Haley, a tea party favorite, however.

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