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Rick Santorum has put so much thought into running a "positive" campaign that Newt Gingrich, in a bitter speech the night of the Iowa caucuses, even complimented him for the way he has sought the GOP nomination.

But after having his record ripped in ads by a super PAC supporting Mitt Romney and by Rep. Ron Paul in South Carolina, Santorum took the unusual step of calling a news conference to give his side of the story, with a tinge of darkness.

In the ads, Santorum, whose hopes in the upcoming South Carolina primary hinge on evangelical and conservative voters, is called a serial earmarker (a big no-no with the Tea Party) and is blamed for letting convicted felons vote. Santorum is swinging back.

At the presser in Columbia today, Santorum said Romney has sent out his "henchmen" in the super PAC, "lied about his record," and "went after earmarks" as governor of Massachusetts. He saved some punches for the rest of the candidates, too: He said Paul is "one of the greatest earmarkers of all time," and that Rick Perry "supported earmarks and, in fact, requested 1,200 of them."

"The people of South Carolina need to know the truth, and when they know the truth, I think they will find it's not surprising that Gov. Romney's not as conservative as his PAC and his campaign has said that he is," Santorum said.

Inevitably, Santorum was asked about his campaign tactics: Is he going to veer onto the low road to swipe at his rivals? He responded by saying no and pledging not to run negative ads, but he didn't hide his anger.

"I'm not going to go out there and misrepresent someone's record for political purposes. I don't do that. I don't believe in it," he said. "The people of South Carolina deserve an honorable campaign, someone who goes out and tells the truth, someone who talks about their record and juxtaposes it with someone else on a factual basis, and we are going to do that, as we have in this campaign, and we're going to keep it above board. We're going to keep it about issues, not about things that clearly are intended to mislead the people of South Carolina."

At one point during his early-morning news  conference, Santorum turned prickly as reporters asked him about the ad about his vote to let felons who have left prison regain the right to vote. Santorum, slightly annoyed, defended his vote by saying that "the federal government has the role to determining who is eligible to vote in federal elections."

"We would not erase the laws with respect to state elections, and, in fact, the state could put together, if they wanted, a system where they would allow people to vote in federal elections and not in state elections," he said.